NIF: added NiStencilProperty record handling

NiStencilProperty appears in Better Clothes plugin. If it not handled,
some parts of NPCs bodies will be not rendered.
Avoid duplicating skeletons due to casing issues

Manually created resource names are apparently always case sensitive, causing
some skeletons to get loaded multiple times.
Share the space with the parent entity only when there's real skinned meshes

The existence of a base skeleton doesn't mean it shares the same bone
structure. If there isn't an actual skinned entity besides the base, simply
attach it to the bone like unskinned meshes should be.
Merge remote-tracking branch 'zini/master' into skeleton-fixes
Fix some issues that seem to have appeared in the CMake configuration.
Only install binaries which are enabled to be built
Merge remote-tracking branch 'chris/skeleton-fixes'
Merge remote-tracking branch 'amdmi3/installation-fixes'
Removed "Unloading Cell..." text from loading screen

OpenMW unloads the cell so fast, it's hardly noticable. This commit gets rid of
the "flicker" every time a cell loads.