Making selections

[snake-biscuits]

We need a way to select objects to apply operations to them.
Since we only have a 3D viewport at time of writing, raycasts seems like the way to go

This will need to work for brushes, displacements, faces & point entities

Work with whoever picks up #6 to ensure the interface to the selection is easy to work with

[snake-biscuits]

This didn't get finished in time for v0.0.4 since it needs a refactor of the way .vmf files are held and modified

[snake-biscuits]

Putting a crosshair in the center of the viewport when flying helps users know where they will select, that they are controlling the camera, and reduces motion sickness.

Adding one sounds like a great idea, can't just change the cursor because it needs to move and teleport to the center for tracking. A custom crosshair would also be nice (.png?)

[snake-biscuits]

Being able to select a brush and view it's source should help with Issue #12
If we can select the clearly subdivided displacements, we can see how subdivision works