replace file chooser with file dialog and add format filter to avatar selector

The file chooser has been deprecated, but can be easily replaced with Gtk.FileDialog.

I took the chance, to add format filters to the avatar selector, as originally, this dialog/chooser did not have them, making it harder to find a file for an avatar, and easier to select a non-image or an image with an unsupported image format that would be not suitable here.

Hence, a format filter for any supported type as the default makes it easier to make an expected input. Also, adding each supported format individually helps users to find an image in cases of directories with many image files.