Total overhaul of actions handling, references #616, fixes #592
- use only numeric identifiers;
- do not rely on actions supplied by QtWebKit (they are often broken under Windows);
- decouple shortcuts from actions;
- create global actions on demand;
- always show keyboard shortcut of action, no matter if it is global or local instance;
- slightly reduced memory usage;
- hopefully no regressions other than keyboard shortcuts configuration being temporarily unavailable.