Splits `Node3DEditorViewport` into a dedicated file.
The main `Node3DEditorPlugin` file was one if the biggest files of the entire engine. It was getting way too big to be viewed, parsed and displayed effectively (e.g. GitHub, certain IDEs). While still big after this PR it at least splits it more or less in half.
- Moves `Node3DEditorViewport` and related helper classes to its own file `node_3d_editor_viewport` file.
- Moves the (shared) constexpr to its own file `node_3d_editor_constants` file.
- Forward declares many more classes.
- Removes unnecessary includes.
- Fixes classes that needed the actual Node3DViewport class.
A number of headers in the codebase included `rendering_server.h` just for
some enum definitions. This means that any change to `rendering_server.h` or
one of its dependencies would trigger a massive incremental rebuild.
With this change, we decouple a number of classes from `rendering_server.h`,
greatly speeding up incremental rebuilds for that area.
On my machine, this reduces incremental compilation time after an edit of
`rendering_server.h` by 60s (from 2m57s).