[appindicator] use AppIndicator3 and AyatanaAppIndicator3

I'm using archlinux and installing libappindicator3 provides gir AyatanaAppIndicator3 and not AyatanaAppIndicator3. I suppose the following code in appindicator_integration/plugin.py would fix this:

import gi
from gi.repository import Gtk, GLib, Gdk

try:
    gi.require_version('AppIndicator3', '0.1')
    from gi.repository import AppIndicator3 as appindicator
    ERRORMSG = None
except:
	try:
		gi.require_version('AyatanaAppIndicator3', '0.1')
		from gi.repository import AyatanaAppIndicator3 as appindicator
		ERRORMSG = None
	except (ValueError, ImportError):
		ERRORMSG = 'Please install libappindicator3'

I could only test this on my system but I suppose that it should not break anything.