[needs-docs][feature] New button for empty the composer table (#5407)

Adds a button to empty a composer table. Useful when you have a table
with a lot of columns and you want only a few.

Also change table  from SingleSelection to MultiSelection and allow to
delete selected rows and move selections up and down.

src/app/composer/qgsattributeselectiondialog.cpp

@@ -280,6 +280,7 @@ QgsAttributeSelectionDialog::QgsAttributeSelectionDialog( QgsComposerAttributeTa
   connect( mColumnUpPushButton, &QPushButton::clicked, this, &QgsAttributeSelectionDialog::mColumnUpPushButton_clicked );
   connect( mColumnDownPushButton, &QPushButton::clicked, this, &QgsAttributeSelectionDialog::mColumnDownPushButton_clicked );
   connect( mResetColumnsPushButton, &QPushButton::clicked, this, &QgsAttributeSelectionDialog::mResetColumnsPushButton_clicked );
+  connect( mClearColumnsPushButton, &QPushButton::clicked, this, &QgsAttributeSelectionDialog::mClearColumnsPushButton_clicked );
   connect( mAddSortColumnPushButton, &QPushButton::clicked, this, &QgsAttributeSelectionDialog::mAddSortColumnPushButton_clicked );
   connect( mRemoveSortColumnPushButton, &QPushButton::clicked, this, &QgsAttributeSelectionDialog::mRemoveSortColumnPushButton_clicked );
   connect( mSortColumnUpPushButton, &QPushButton::clicked, this, &QgsAttributeSelectionDialog::mSortColumnUpPushButton_clicked );
@@ -330,13 +331,12 @@ QgsAttributeSelectionDialog::~QgsAttributeSelectionDialog()
 
 void QgsAttributeSelectionDialog::mRemoveColumnPushButton_clicked()
 {
-  //remove selected row from model
-  QItemSelection viewSelection( mColumnsTableView->selectionModel()->selection() );
-  if ( viewSelection.length() > 0 )
-  {
-    int selectedRow = viewSelection.indexes().at( 0 ).row();
-    mColumnModel->removeRow( selectedRow );
-  }
+  //remove selected rows from model
+  QModelIndexList indexes =  mColumnsTableView->selectionModel()->selectedRows();
+  int count = indexes.count();
+
+  for ( int i = count; i > 0; --i )
+    mColumnModel->removeRow( indexes.at( i - 1 ).row(), QModelIndex() );
 }
 
 void QgsAttributeSelectionDialog::mAddColumnPushButton_clicked()
@@ -348,23 +348,23 @@ void QgsAttributeSelectionDialog::mAddColumnPushButton_clicked()
 void QgsAttributeSelectionDialog::mColumnUpPushButton_clicked()
 {
   //move selected row up
-  QItemSelection viewSelection( mColumnsTableView->selectionModel()->selection() );
-  if ( !viewSelection.empty() )
-  {
-    int selectedRow = viewSelection.indexes().at( 0 ).row();
-    mColumnModel->moveRow( selectedRow, QgsComposerAttributeTableColumnModelV2::ShiftUp );
-  }
+
+  QModelIndexList indexes =  mColumnsTableView->selectionModel()->selectedRows();
+  int count = indexes.count();
+
+  std::reverse( indexes.begin(), indexes.end() );
+  for ( int i = count; i > 0; --i )
+    mColumnModel->moveRow( indexes.at( i - 1 ).row(), QgsComposerAttributeTableColumnModelV2::ShiftUp );
 }
 
 void QgsAttributeSelectionDialog::mColumnDownPushButton_clicked()
 {
   //move selected row down
-  QItemSelection viewSelection( mColumnsTableView->selectionModel()->selection() );
-  if ( !viewSelection.empty() )
-  {
-    int selectedRow = viewSelection.indexes().at( 0 ).row();
-    mColumnModel->moveRow( selectedRow, QgsComposerAttributeTableColumnModelV2::ShiftDown );
-  }
+  QModelIndexList indexes =  mColumnsTableView->selectionModel()->selectedRows();
+  int count = indexes.count();
+
+  for ( int i = count; i > 0; --i )
+    mColumnModel->moveRow( indexes.at( i - 1 ).row(), QgsComposerAttributeTableColumnModelV2::ShiftDown );
 }
 
 void QgsAttributeSelectionDialog::mResetColumnsPushButton_clicked()
@@ -374,6 +374,13 @@ void QgsAttributeSelectionDialog::mResetColumnsPushButton_clicked()
   mSortColumnComboBox->setCurrentIndex( 0 );
 }
 
+void QgsAttributeSelectionDialog::mClearColumnsPushButton_clicked()
+{
+  //remove all columns
+  mColumnModel->removeRows( 0, mColumnModel->rowCount() );
+  mSortColumnComboBox->setCurrentIndex( 0 );
+}
+
 void QgsAttributeSelectionDialog::mAddSortColumnPushButton_clicked()
 {
   //add column to sort order widget

src/app/composer/qgsattributeselectiondialog.h

@@ -123,6 +123,7 @@ class QgsAttributeSelectionDialog: public QDialog, private Ui::QgsAttributeSelec
     void mColumnUpPushButton_clicked();
     void mColumnDownPushButton_clicked();
     void mResetColumnsPushButton_clicked();
+    void mClearColumnsPushButton_clicked();
     void mAddSortColumnPushButton_clicked();
     void mRemoveSortColumnPushButton_clicked();
     void mSortColumnUpPushButton_clicked();

src/ui/composer/qgsattributeselectiondialogbase.ui

@@ -77,6 +77,13 @@
            </property>
           </widget>
          </item>
+         <item>
+          <widget class="QPushButton" name="mClearColumnsPushButton">
+           <property name="text">
+            <string>Clear</string>
+           </property>
+          </widget>
+         </item>
         </layout>
        </item>
        <item row="1" column="1">
@@ -113,7 +120,7 @@
           <enum>Qt::IgnoreAction</enum>
          </property>
          <property name="selectionMode">
-          <enum>QAbstractItemView::SingleSelection</enum>
+          <enum>QAbstractItemView::MultiSelection</enum>
          </property>
          <property name="selectionBehavior">
           <enum>QAbstractItemView::SelectRows</enum>