Action enhances QAction with a subset of KDE's KAction capabilities

Action enhances QAction with a subset of KDE's KAction capabilities

This mostly adds configurable default and custom shortcuts to QAction. The API
we use has been taken from KAction, in order to make KDE integration easier later on.
Note that we only define/implement a subset of KAction's API, since we don't have
global shortcuts and don't support alternate shortcuts (yet?).