Moved Opacity logic to GuiComponent

Moved Opacity logic from ImageComponent to GuiComponent so any extender
of GuiComponent che implement its opacity logic.

Implemented Opacity logic for TextComponent (now text can have fade
animation)
Merge branch 'master' of https://github.com/leandromacrini/EmulationStation
Merge pull request #92 from leandromacrini/master

Moved Opacity logic to GuiComponent
Changed AnimationComponent to use GuiComponent instead of ImageComponent.
Possible now that the opacity logic has been moved to GuiComponent.
Merge remote-tracking branch 'upstream/master'