DolphinMainWindow: show a banner when the user presses the shortcut of a disabled...

DolphinMainWindow: show a banner when the user presses the shortcut of a disabled action

Currently, there's no feedback when the user presses a shortcut of a disabled
action, e.g. cut / paste in a read-only directory. This patch shows a banner
in that case.

It's implemented by enabling a QShortcut for each disabled action. the
QShortcut is deleted when the action is enabled again.

The following actions are included:
cut
paste
rename
moveToTrash
deleteWithTrashShortcut
deleted
createDir
copyToOtherView
moveToOtherView