ESourceRegistry: Use g_main_context_ref_thread_default().
ESourceRegistry: Use g_main_context_ref_thread_default().

Slightly more convenient than g_main_context_get_thread_default(), which
requires checking for NULL before referencing the main context ourselves.

matzipan/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,Distrotech/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,tintou/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,Distrotech/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server