Themes: Remove listener if exception is thrown

If something goes wrong and a client does not remove itself, the
theme manager may still have a reference to the listener and an
exception is thrown while trying to call one of the listener's methods.
When this happens, we should not only log it but remove the bad listener.
[1/2] SystemUI: Add time-context headers to the notification header

This commit adds image headers to the notification panel header,
based on the date/time. There are pictures for morning, afternoon
and night, as well as special events like Christmas and new
year's eve.
More will be coming, with eventually a way to use an external
(as in, Internet) provider for those pictures.