Mecha fixes

Prevents mech actions from acting on the user's inventory.  Fixes #1487.
(Almost) fully prevents inventory use from within a mech, to be more consistent.  The internals GUI button still works on the theory that it's emergency equipment.
Mecha HUD hiding

Hides the user's inventory (as per F12) when they enter a mecha, since it is essentially unusable.  Pressing F12 will reveal it, as will exiting the mecha.  If your inventory is hidden before you enter the mech, it will remain hidden when you leave.
More correct fix for #1164 that does not introduce new bugs (oops)
Re-adds the ability for mobs to turn when clicking on a wall object in their square (APC, etc)
Re-adds ghost shift- and alt-clicking, sorry about that.
Merge pull request #1589 from SuperSayu/clickstuff3

Mecha, hud, and click fixes