[needs-docs] Reword "delete" option in db connection dialog

Button has been changed to "remove", to make it clear that
the button just removes the connection, it does not delete
the database! (It was confusing for spatialite dbs)

Also add tooltips to buttons for extra clarification.

src/ui/qgsdbsourceselectbase.ui

@@ -15,8 +15,7 @@
   </property>
   <property name="windowIcon">
    <iconset>
-    <normaloff/>
-   </iconset>
+    <normaloff>.</normaloff>.</iconset>
   </property>
   <property name="sizeGripEnabled">
    <bool>true</bool>
@@ -38,29 +37,41 @@
        <layout class="QHBoxLayout" name="horizontalLayout">
         <item>
          <widget class="QPushButton" name="btnConnect">
+          <property name="toolTip">
+           <string>Connect to selected database</string>
+          </property>
           <property name="text">
            <string>Connect</string>
           </property>
          </widget>
         </item>
         <item>
          <widget class="QPushButton" name="btnNew">
+          <property name="toolTip">
+           <string>Create a new database connection</string>
+          </property>
           <property name="text">
            <string>New</string>
           </property>
          </widget>
         </item>
         <item>
          <widget class="QPushButton" name="btnEdit">
+          <property name="toolTip">
+           <string>Edit selected database connection</string>
+          </property>
           <property name="text">
            <string>Edit</string>
           </property>
          </widget>
         </item>
         <item>
          <widget class="QPushButton" name="btnDelete">
+          <property name="toolTip">
+           <string>Remove connection to selected database</string>
+          </property>
           <property name="text">
-           <string>Delete</string>
+           <string>Remove</string>
           </property>
          </widget>
         </item>
@@ -158,7 +169,16 @@
       <bool>false</bool>
      </property>
      <layout class="QGridLayout">
-      <property name="margin">
+      <property name="leftMargin">
+       <number>9</number>
+      </property>
+      <property name="topMargin">
+       <number>9</number>
+      </property>
+      <property name="rightMargin">
+       <number>9</number>
+      </property>
+      <property name="bottomMargin">
        <number>9</number>
       </property>
       <property name="spacing">