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.
OPENGL: Fix linear filtering when the texture size doesn't match the real size
Merge pull request #302 from clone2727/gl-linear-fix

OPENGL: Fix linear filtering when the texture size doesn't match the real size
VIDEO: Hook some more of our ADPCM decoder variants to our AVI video decoder

Information for the AVI audio track format IDs has been taken from libav.
Thanks to clone2727 for his great help on this.
FLUIDSYNTH: Add separate dialog for FluidSynth settings

I don't really understand what these parameters do, or what the
sensible values are, so for now the sliders are limited only by
the allowed (or, in one case, "safe") values.
BADA: Add completely untested layout changes to Bada theme

I've simply copied this from classic_layout.stx. It probably needs
a bit of tweaking.
GUI: Make the FluidSynth settings dialog a bit more like Qsynth

To help people familiar with Qsynth (I'm not, but it seems to be
one of the more polished FluidSynth front ends), use the same
presentation and terminology for the FluidSynth settings.

More to follow.