Switch to HdyApplicationWindow
This is the first part of !814 (merged)
DzlApplicationWindow
will be problematic for the new tabs since it expects all window-wide shortcuts to be managed through its machinery. That works for the application shortcuts, but not shortcuts that come from libhandy. This can already be seen with the fact we have to override Ctrl+Tab
manually.
Handling fullscreen ourselves also will allow us to have tabs in fullscreen with !814 (merged).
Fixes #1277 (closed)