[FEATURE REQUEST] Allow to implement gio::Action

[andy128k]

I'd like to be able to implement my own types of actions, similar to what Gio does itself with g_settings_action internally.

My use case is a proxy action which forwards to another action (g_settings_action) but:

• has a different name (I'd like to preserve GSchema);
• inverts boolean state (a setting is "hide-something" but user action which makes sense is rather "something-visible");
• maps parameters to/from int32/string.

[sdroege]

That looks relatively easy to support, the interface doesn't contain any strange virtual methods. Do you want to give it a try?