Enable caching on a call to atspi_accessible_set_cache_mask

Enable caching on a call to atspi_accessible_set_cache_mask

Previously, caching was always disabled unless atspi_event_main() was
called to start a main loop, since we need to handle the case of a
simple application/script that performs an action and then sleeps rather
than entering a main loop. In such a case, libatspi will not be able to
detect that an event has been fired to update the cache, so caching
should be disabled. However, an application may want to start a main
loop through some other mechanism than by calling atspi-event_main(),
and caching should be done in such a case. So we'll assume that, if
atspi_accessible_set_cache_mask is called, then the application expects
caching to work and will run a main loop one way or another. Note that
it is possible for a script to run a main loop in some instances but not
others, and, in such a case, if it is not using atspi_event_main(), then
it should set caching to ATSPI_CACHE_NONE when not in a main loop and
back to ATSPI_CACHE_DEFAULT when running a main loop.