Add a loading screen to the entity editor

[tchx84]

Opening complex Entities with the Entity Editor can take a couple seconds. When the loading occurs, since there's no loading screen, the application freezes. Which is a pretty bad experience.

There are two complementary solutions for this:

1. Move the expensive operations (basically, pre-processing of the animations) to a separate thread.
2. Add a proper loading screen to that section while the pre-processing occurs.