Add AvatarSelector

This implements an AvatarSelector widget and a CropArea widget.

Images can be loaded via drag and drop or an external 'Select Picture' button > FileChooser > path. The crop frame can be moved and resized.

• get_avatar_surface yields a surface for use within Gajim
• get_avatar_bytes yields bytes ready for publishing

Original code of the CropArea widget was ported from GNOME’s Cheese application um-crop-area.c.