TMR: Change how we highlight UI elements

[BorisJov]

(TMR = Tutorial Manager Refactor)

Currently we highlight UI elements by basically copying them on top of the area faded out by use of the Tutorial Popup. Instead, Alex has suggested we use UnmaskForUGUI, which solves the problem of the UI being faded out by unmasking/making a hole through to the top of the layers. This should be implemented as part of the Tutorial Manager Refactoring.