WIP: Unify GtkFileChooserDialogue

This is till very much WIP.

Current state:

Things, that do not work yet:

1. The "Preview Snapshot" does not show the new headerbar buttons yet
2. Glade still exports the old dialogue when saving
3. We still have the old GtkButtonBox in the footer of the window
4. When changing the GtkFileChooserAction, the Open/Save/Create/Select button does not get updated

#419