gmain: Simplify source id tracking

Simplify our tracking of issued source id integers and fix some bugs.

Previously the source's id was remove from the 'used' table from
source_remove_from_context() which was also called if the source
priority was changed (in which case it would never be added back to the
table).  The source id could be reissued in that case.

In the new approach, we just always keep a hash table of sources, by
source id.  This simplifies the logic and will also allow us to improve
performance of g_main_context_find_source_by_id() which is called in some
fairly common cases, such as g_source_remove().  These improvements will be in
the following commits.

https://bugzilla.gnome.org/show_bug.cgi?id=724839
gmain: simplify g_main_context_find_source_by_id()

Since we now keep a hashtable of sources, we can implement this function
without iteration.

https://bugzilla.gnome.org/show_bug.cgi?id=724839