Context-Sensitive Cursor

The mouse cursor icon now changes depending on what it's hovering above
in the game.

There are four base cursor types:-
Valid - For when the mouse is over a walkable area in-game or over the
GUI.
Invalid - For when the cursor is over a non-walkable area in-game.
Use - When the cursor is over a friendly character in-game.
Hostile - When the cursor is over a hostile character in-game.
There are alternate versions of each cursor type for when the player has
an ability selected to make it easier to tell when an ability is
selected or not.