Use thunar-launcher / thunar-menu for tree-view

... instead of foreign menu creation.

This MR will fix different things by using thunar-launcher/thunar-menu for the tree view:

• Fix missing UCA support for tree-view context menu on current master (Issue #349 (closed)), regression introduced by !10 (merged)
• Drop some more (20?) G_GNUC_BEGIN_IGNORE_DEPRECATIONS
• Remove duplicated menu-creation code ()

To prepare the launcher for some new use-cases, I first had to fix a hand full of issues/inflexibilities of it.