Editor: Refactor usage of default camera.

[Mugen87]

The PR slightly refactors the usage of the editor's default camera. It ensures

• that the fixes of 4abbce4 still work
• but handles DEFAULT_CAMERA now as a constant
• it's also a module scope variable now and does not leak anymore in Viewport

This is done by introducing a new signal cameraResetted that notifies Viewport to update the aspect ratio of the editor's camera.

[Mugen87]

BTW: I would like to try adding support for an orthographic editor camera similar to Unity's scene editor. This PR and another follow-up change prepare the codebase so it's easier to implement this feature.

[mrdoob]

Thanks!