GRAPHICS: Serialize PixelFormat for thumbnails and allow 4Bpp surfaces.

This allows any 2Bpp/4Bpp Surfaces to be written via saveThumbnail
and loadThumbnail now. It furthermore will preserve the loaded Surface
in loadThumbnail.
SCUMM: Silence C++11 narrowing warnings.
SKY: Silence C++11 narrowing warnings.
AGI: Silence C++11 narrowing warning.
PEGASUS: Silence C++11 narrowing warnings.
SAGA: Silence C++11 narrowing warnings.
SWORD25: Silence C++11 narrowing warnings.
OPENGL: Silence C++11 narrowing warning.
SDL: Silence C++ narrowing warnings.
VIDEO: Silence C++11 narrowing warnings.
AUDIO: Silence C++11 narrowing warnings.
CONFIGURE: Add option to build as C++11.
COMMON: Add a header for C++11 keywords for pre-C++11 compilers.

This currently adds defines for nullptr and override, so we can use them in
pre-C++11 compilers, while taking advantage of them with C++11 compilers.
KYRA: Use override in subclasses of Sound, which override voicePlay.
WINTERMUTE: Guard ConfMan a bit better.
WINTERMUTE: Mark drawLine as unused with dirty-rects.
WINTERMUTE: Store larger thumbnails (Bump Savegame-version to 1.2.0.0)
WINTERMUTE: Silence a warning.
WINTERMUTE: Allow users to enable the FPS-counter in the GUI, and disable MT32-settings.
WINTERMUTE: Use prefix-operators on iterators.