Entangle 'thunar_util_parse_parent' (#1632)
- Use a separate, dedicated method to find a
GtkWindow
(do one thing!) - Make sure that no transient parent is defined for dialogs, if a specific screen is requested
- Unify API of
ThunarApplication
methods to use a generic "parent" parameter instead of a screen
Fixes #1632 (closed) while not breaking the fix of #1591 (closed)