Merge lp://qastaging/~charlesk/libindicator/add-visibility into lp://qastaging/libindicator/0.5

Since the libindicator release is being pushed until next week, I'd like to defer this merge until early next week -- I see a couple of things in r437 that I'd like to improve before the merge.

Some fixes needed:

* I think the param spec for default visibility should be a boolean instead of a string.
* entry_get_private should be static
* default /*ENTRY_INIT*/ should actually be case ENTRY_INIT with the default case being a g_warn_if_reached() for better error reporting.
* It seems like indicator_object_set_visible() should set the value in the private structs of the indicator entries so that if get_entries is called after calling it, the correct list would be returned.
* The signal connect for on_settings_changed() should use a detail string since it's only looking for a single value, so "changed::visible"
* schema_set() needs to (sadly) check to make sure that the schema exists otherwise GSettings will dump the application. You can use g_settings_list_schemas to discover that.

I'm a little concerned about the "limbo code" in that I think it should be the responsibility of the subclassing indicator to handle the reference counting. If we handle it in the superclass, we're always going to end up in the situation where the subclasses don't really know how to completely unref an object. It seems like we should fix that issue there.

Also, with the gsettings code I think this is a good use case for the GSettings paths. So we could make the actual schema in the libindicator package, but without a path. Then we allow the subclassing indicators define the path that the schema exists at. This would give us more guarantee of the entries in the schema and would make it easier for upgrades to that.

Oh, it also needs a test. Or Allan comes to your house and hugs you until you write one :-)