Allow locking/unlocking the mouse through the input plugin

[RiscadoA]

Problem

Currently, to implement, for example, a FPS camera, to 'hide' the mouse and lock it to the center of the window, you need to access the Window class itself.
The logic is also a bit weird to mess around with, and it seems like something the input plugin should handle by itself, specially with #1399.

Solution

Maybe we should do something like input.lockMouse() and input.unlockMouse(), and make it so that the mouse axis only report movement when the mouse is locked.