Merge: app::ui: intro Label and apply many other fixes

Most features are self-explanatory, but here it is anyway. `Label` defines a simple non-editable text field. And the newly public `remove` method remove views from other views (such as `Layouts`).

Other fixes clean up the relation with the Java GC and multi-threads behavior.

Pull-Request: #1827
Reviewed-by: Jean Privat <jean@pryen.org>