Move avatar icon generation from mailnag-gnome-shell to a mailnag plugin

[pulb]

Move the avatar icon generation code from mailnag-gnome-shell (aggregate-avatars.vala) to a mailnag plugin.

• The generated icons should be stored in a volatile location like /run/user/<current_user>/mailnag/avatars.
• Icons should have the name of the associated email address.

Benefits:

• The mailnag-gnome-shell extension published to extensions.gnome.org currently does not support avatar icons (because uploading binaries like aggregate-avatars is not allowed). This would change if mailnag itself would provide avatar icons.
• The libnotify plugin could show those icons in a notification.
• Other extensions/programs could use the generated icons.
• mailnag-gnome-shell would be javascript only (no more compilation/installation steps required).

[pulb]

Please see also #139

[pulb]

This should be useful: https://github.com/hugosenari/folks