[gif] - add proper gif support and support for animated splash screens

This is basically taking alot from #4474 - squashed it down alot and added support of loading animated Splash.gif when found in the users media folder.

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.

@jmarshallnz you already started to review to original PR and suggested a rewrite into IImage or so - this work here is based on the original pr - so its still not using the backgroundloader ...

Also this changes the static 1280x720 rendersize and uses current resolution instead...

@ace20022 i left all the fileitem and picture stuff out which is not needed for the splash rework ... just fyi
if you require libgif5 then you must check for a proper version. Since this lib doesn't seem to provide a pkg-config file, you'll have to check the header...
Oh and the unavailability of v5 on even unreleased ubuntu 14.10 is not a good start either