Add FindSDL2 and use it. Code not updated for SDL2 yet, compile fails. Hacks for OS X ported from old FindSDL (not sure if they are still required).
Change Duet logic so Performous doesn't crash anymore.

This changes the internal logic a bit: Since the Engine always wants the
same number of Analyzers as Vocals, we always use as much Analyzers as
vocals. In the case the user either selects "Normal Mode" from the Duet
menu or the song does not contain a duet in the first place, we just
assign the first selected vocal track (or the only one) to all
microphones.

TODO: The duet menu should somehow visualize when "Normal Mode" is
selected, that only the first vocal selection is relevant.
Show microphone icons in duet menu.

This still needs a little bit of layout adjusting...
Move microphone colors into a central place.

These colors were defined in 3 places. Does this need a more efficient
implementation?
have it linking agains SDL2 libraries in stead of SDL1 libs
Merge remote-tracking branch 'origin/master' into sdl2

Conflicts:
	game/CMakeLists.txt
resolved input issues, now hits video stuff at 25% compiling.
video stuff SDL2 porting
final adjustments to SDL2 port, now only linking errors remain
fixed linking error
Program now actually RUNS!!!
don't use a renderer (scaling still broken :-( )
fullscreen is not 640x480 anymore, but still black screen...
fixed fullscreen bug, context is not destroyed anymore, resize event is
not triggered
forgot some comments
fixed resizing (in an ugly way, but window& context isn't triggered
anymore in resize() so it shouldn't be a problem)
ugly fix for window beeing too small
windows hack
resolved textinput issue
Make microphone icon a bit smaller.