The current version of GTimer will only return the time elapsed since the
last call to g_timer_start().  Thus repeated calls to g_timer_start() and
g_timer_stop() on the same instance of a GTimer will not result in the
total elapsed time being returned by g_timer_elapsed().

This patch adds the following:

- g_timer_total_elapsed() : returns the total elapsed time that the timer
has been running since g_timer_new() or g_timer_reset() was called
- g_timer_is_active() : returns whether or not the timer is currently running
- doc strings for gtkdoc have been added to all functions
- The existing behaviour is preserved