Use init and clear for GMutex and GCond

- g_mutex_init() replaces g_mutex_new()
- g_mutex_clear() replaces g_mutex_free()
- g_cond_init() replaces g_cond_new()
- g_cond_clear() replaces g_cond_free()

The old functions were deprecated and the new functions were added as
of GLib 2.32.

This requires all structures to use GMutex members instead of GMutex *
pointers; likewise for GCond. Since we cannot set the GMutex * pointers
to NULL anymore when the ChimaraGlk widget is finalized, we add an
after_finalize flag that is set at the beginning of
chimara_glk_finalize().