Infinite mouse movement for S R shortcuts

[kaixoo]

Implements infinite mouse movement like in blender for shortcuts like S R and, perhaps, inputs.

Fixes #342

I could sketch this code while I figure out QCursor's API and try to compile Qt 5.15.16. Am I in the good direction or is this overcomplicated?

[rodlie]

1. src/ui is a library, your code must be a proper class
2. Should this be a class?
3. How is this going to work in Friction?

PR's should be implemented enough to be usable/testable.

[kaixoo]

I thought of using a Singleton at first but it got too complicated. Maybe it should be one tho. This is just a barebones while I try to compile Qt and actually test the code, so you can review if it's worth it.

How does it work? As a Singleton, you'd change the cursor's state from e.g. FrictionCursor::get_instance().set_free() depending on what you want. If you are using the S R shortcuts, you'd change to limited mode. If you are modifying an input box, you'd change to locked mode.