* 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)
Mixing ForkAwesome icons too much with the regular icon theme doesn't look
good (depending on the theme) so let's prefer ForkAwesome icons within the
tray widget which already uses ForkAwesome in some places like certain
buttons and models. This makes it also look more in-line with Syncthing's
web UI. For context menus and dialogs let's prefer system icons because
there are hardly any ForkAwesome icons used/required so far and it is maybe
nicer to be in-line with the system here.
* Use the name as sorting criteria and fall back to the ID if there's no
name
* Use new SyncthingSortFilterDirectoryModel also for Plasmoid's filtering
* See https://github.com/Martchus/syncthingtray/issues/75
