New shortcut for Copy with Headers and some self-documented shortcuts

Ctrl+H invokes the Copy with Headersers action in the Extended Table Widget
as suggested in #1058.

The following options in context menu display now their shortcuts:
Set to Null, Copy, Copy with Headers and Paste.

Some other shortcuts are now displayed in the corresponding tooltips.

src/EditDialog.ui

@@ -160,8 +160,8 @@
         <rect>
          <x>0</x>
          <y>0</y>
-         <width>83</width>
-         <height>40</height>
+         <width>98</width>
+         <height>35</height>
         </rect>
        </property>
        <layout class="QVBoxLayout" name="verticalLayout_2">
@@ -212,6 +212,9 @@
      </item>
      <item>
       <widget class="QPushButton" name="buttonApply">
+       <property name="toolTip">
+        <string>Apply data to cell [Ctrl+Return]</string>
+       </property>
        <property name="text">
         <string>Apply</string>
        </property>