The functionality of `push_unhandled_input` has changed so that it no
longer propagates input events to SubViewports.
This makes it less predictable and it should be deprecated in favor of
`push_input` which provides the same functionality and more.
Also this deprecation simplifies the Viewport-API by reducing the methods
for pushing input events, so that users don't need to worry about when
to use which function in order to insert input events.
This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
* `_gui_input`, `_input`, `_unhandled_input` and `_unhandled_key_input` are now regular C++ virutal functions.
* Everything else converted to GDVIRTUAL
* BIND_VMETHOD is gone, always use the new syntax from now on.
Creating `_gui_input` method and using the binder to register events will no longer work, simply override the virtual function now.
