[th/gsource-attach] change/fix priorities for various idle GSources and use NM convenience wrappers

g_idle_add() uses G_PRIORITY_DEFAULT_IDLE priority (unlike most other methods, that default to G_PRIORITY_DEFAULT. Adjust some cases where we schedule idle action to use the lower priority (as g_idle_add() would use). We probably used there a higher priority as a mistake, not intentionally.

Also, we now have convenience wrappers like nm_g_idle_add_source() and nm_g_timeout_add_source(), which make the common operation of adding a source to the default context simpler. Use those were the behavior is the same.