Implement a Selector API in Masonry

[PoignardAzur]

Right now, our only way to get a specific widget in the widget tree is id-based methods.

This requires us to create widgets with pre-allocated IDs, which is cumbersome in a couple of ways. And because WidgetId carries no type information, we need to downcast the widget we retrieve this way.

A better API would be one based around a typed Selector<W> type; search methods could take that type as an input (which could have optional attributes like id, substring, etc), and would returned a typed WidgetRef/WidgetMut with the downcast done internally.

Doing so would probably make a lot of unit tests easier to read and write.