Allow to additionally import "AppIndicator3" to "AyatanaAppIndicator3"

nbxmpp does this already for us

There was an API change in python-axolotl

This fixes some deprecation warnings related to Gtk.Color and restructures
code and formatting according to pylint suggestions

If Open Link in Browser is selected as default action for left clicking an image,
the user would get a warning when clicking an encrypted image. Fall back to open
image directly instead of issuing a warning dialog.