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_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.