New classes: SoundStream and Display::Resolution. A few improvements. - Created SoundStream class to play sound streams which loads data from file on demand instead of buffering it all on the memory. - Added Display::Resolution utility class with data regarding common display resolutions. - Added Sprite copy constructor with option to choose whether to manage sheet or not. Now its possible to copy a Sprite while choosing to enable/disable the sheet deletion flag of the copied one. This allow shallow copies of sprite. - Refactored both SFML adapters to exchange pointer usages with direct instance usages, reducing indirection. SFML don't need an global init function and, thus, don't require pointer usage when dealing with its classes. - Fixed Allegro 5.0 adapter's Sound::set/getPlaybackSpeed when streaming. - Removed leftover references to the old Icon class. - Adding note regarding adapter-dependent behavior on Sound class.