Added a cmake option to disable keyboard grabbing on linux. Enabled by default.

Fixes #293
Added support to build Crypts.

Closes #314

I also upgraded the models format and tested the editor to make sure.
Made the keyboard grab option a linux option only.

And properly defined it through CMake Cxx Flags.

Thanks to @akien-mga for pointing out.
Merge pull request #317 from Bertram25/development

CMake linux option not to grab keyboard + Added support to build a crypt
Changed the KnightCoffin material to use the same texture as the Knight statue.

Since they were using the same bits anyway.
I didn't used the model given by Danimal, since it was the same model in an older format.
Fixed adding Crypts in the Editor.

Fixes #320
Merge pull request #321 from Bertram25/boulder

Some quick-fixes
Merge branch 'development' of github.com:OpenDungeons/OpenDungeons into development