QMessageBox: shortcuts for selectAll, copy don't conflict with buttons

You can hit a key with no modifier to select a dialog button that has
the corresponding mnemonic, but it doesn't make sense to do that if
the Control or Meta key is being held.  Now you can hit Ctrl-A, Ctrl-C
to select all the message detail text and copy it to the clipboard,
even if the dialog has buttons with mnemonics like &Continue and
Save &Again.  But as before, Alt-C, plain C, Alt-A and plain A select
the mnemonic buttons on the dialog.

Change-Id: I40c052dbb7730bde444ac2de405724a3203d1472
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
QMessageBox: implemented shortcut for selectAll

The context menu all along has indicated that Ctrl-A selects all the
text, so now that is actually implemented.

Task-number: QTBUG-6731
Change-Id: I82dac3aea530e5a3fcd59217c4c0118025061c7d
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
standarddialogs example: add detailed text

Demonstrates that you can have mnemonic buttons which don't
conflict with the Ctrl-A and Ctrl-C shortcuts.

Task-number: QTBUG-6731
Change-Id: I41ce64e6f3bd0cf387af13c5c37894b721b5a3bb
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>