Hide elements of the HUD (health/magicka/stamina bars,
minimap) when the corresponding windows (stats/map) are pinned.
Rearrange the remaining hud elements in such cases (like in the
original Morrowind).
Create WindowPinnableBase class for windows which should be
allowed to be pinned.
Add skin for pinnable windows - currently just a copy of normal
window with 1 extra button (hopefully this can be improved later).
Handle clicking on PinToggle button (pinning/unpinning a window).
