[style dock] use a QLineEntry for layer title (#3141)

src/app/qgsmapstylingwidget.cpp

@@ -62,7 +62,7 @@ void QgsMapStylingWidget::setLayer( QgsMapLayer *layer )
 {
   if ( !layer || !layer->isSpatial() )
   {
-    mLayerTitleLabel->clear();
+    mLayerTitle->setText( QString() );
     mStackedWidget->setCurrentIndex( mNotSupportedPage );
     return;
   }
@@ -177,7 +177,7 @@ void QgsMapStylingWidget::updateCurrentWidgetLayer()
 
   mUndoWidget->setUndoStack( layer->undoStackStyles() );
 
-  mLayerTitleLabel->setText( layer->name() );
+  mLayerTitle->setText( layer->name() );
 
   if ( layer->type() == QgsMapLayer::VectorLayer )
   {

src/ui/qgsmapstylingwidgetbase.ui

@@ -154,24 +154,18 @@
        <item row="0" column="0" colspan="2">
         <layout class="QHBoxLayout" name="horizontalLayout">
          <item>
-          <widget class="QLabel" name="mLayerTitleLabel">
+          <widget class="QLabel" name="textLabel">
            <property name="text">
-            <string/>
+            <string>Layer name</string>
            </property>
           </widget>
          </item>
          <item>
-          <spacer name="horizontalSpacer">
-           <property name="orientation">
-            <enum>Qt::Horizontal</enum>
-           </property>
-           <property name="sizeHint" stdset="0">
-            <size>
-             <width>40</width>
-             <height>20</height>
-            </size>
+          <widget class="QLineEdit" name="mLayerTitle">
+           <property name="readOnly">
+            <bool>true</bool>
            </property>
-          </spacer>
+          </widget>
          </item>
          <item>
           <widget class="QCheckBox" name="mLiveApplyCheck">