More Dialogs

I have done some small features and fixes at various ends.

Features:
- GtkDialog, GtkAboutBox, GtkMessageDialog
- gdk: keyval
- GtkAccelGroup (for keyboard shortcuts)
- Implement `user_action` as a (public) replacement for `begin/end_user_action`
- Some text tag functions. Not sure about the names and the underscore. Therefore currently not exposed
- Improve GtkBuilder constructor so that it can also take strings
- Navigation functions (up,down, prev, next) for GtkTreePath

Fixes:
- Accessors for GtkMenu and GtkMenuItem were missing
- In getproperty for GtkTextIter the `_get_` was missing in the call for `:chars_in_line` and `:bytes_in_line` 

Then I have been playing around a little with GtkApplication. But this stuff is not functional yet.