Updated CMakeLists and added DocumentManager.

- Build Mac OS X bundle for TrenchBroom
- Add application and document icons to bundle
- Add DocumentManager class and some tests
- Move MapDocument to View namespace

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>
Fixed several compiler warnings.

Signed-off-by: Kristian Duske <kristian.duske@gmail.com>