(re)add mechanism for a callback in any glib event loop executed before the loop "executes"

This is based on code from earlier commits that were later reversed, but we need some mechanism
to ensure that threads have a thread local tempo map ptr set. The big difference is that this
time we do not implement this for all instances of an AbstractUI - implementation is left to
each thread/event loop

libs/pbd/wscript

@@ -51,6 +51,7 @@ libpbd_sources = [
     'file_archive.cc',
     'file_utils.cc',
     'fpu.cc',
+    'glib_event_source.cc',
     'id.cc',
     'locale_guard.cc',
     'localtime_r.cc',