Add:core: event_call_callback infrastructure|

Add:core: event_call_callback infrastructure|
To be used from threads that want to call the callback
from the main(gui) thread. Users must provide
synhronization of the callback data.
Difference with using timeouts are:
Threads can have own timers
Timer events are low priority, this must be called ASAP.

git-svn-id: https://navit.svn.sourceforge.net/svnroot/navit/trunk@1674 ffa7fe5e-494d-0410-b361-a75ebd5db220