[gif] - support for animated splash screens

Based on #5180 this adds support for animated splash screens.

I don't really think i have done the refactor of splash.cpp right - i just am not into gui stuff :(. But in case we somehow want to support animated splash screens for helix (because their were some animated ones in the logo suggestion thread in the forum) - i think this could be a base to work on it.

Also the rendering of the splash is done in our normal application render loop - this means XBMC is loading up in the background while the animation is playing.