Use FileDialog for all file/path pickers

[penguinpee]

• Use FileDialog throughout
• Handle whatever GLib.Error throws our way
• Handle it the same way regardless of dialog

[penguinpee]

Gtk.FileChooserNative causes a segfault in GTK_IS_FILE_SYSTEM_MODEL when launching the Working Directory dialog (__choose_cwd()).

This aligns __choose_cwd() with __choose_pre_script() and __choose_post_script().

[TheEvilSkeleton]

Is it okay for you run pre-commit run --all-files and squash these commits? If not, I can do it

[penguinpee]

I don't have pre-commit set up.

However, I could squash locally and (force) push to my PR branch, if that helps.

[TheEvilSkeleton]

Don't worry, I'll squash and run pre-commit against it ^^

[TheEvilSkeleton]

Thanks a lot for your contribution :)