* Remove experimental pinning feature again and instead allow using a
normal window
* Pinning made it inconvenient to close the (frameless) window again
* Pinning required hiding/showing the window which didn't look very
nice (and setting flags directly via `QWindow` didn't work as well)
* As normal application/window positioning issues on Wayland are less
problematic (and those aren't going to be fixed any time soon, if at all)
This includes refactoring TrayWidget::applySettings() which
is now a member function and only affects the current instance.
This prevents applying settings on every instance every time
a new tray widget is created.
To apply settings for all tray widgets, the static function
TrayWidget::applySettingsOnAllInstances() is used instead.