Add an optional inactive window for the mouse

The mouse will be active only if it's outside the window centered around the cursor, of `width = window.innerWidth * this.mouseInactiveWindow`, and respective height.

`mouseInactiveWindow` should be in the interval `[0, 1)`, and its default value is 0. That is, it's active everywhere by default, same as before.

This modification aims at enlarging the "non-moving area" of the mouse, and thus, helping the user to stand still, or move just with the keyboard.