Made of libksirtet a "real" library. Headers are now installed into the "include" directory and can be used to compile other games.

This change was needed to let KFoulEggs and Klickety compile out of the "kdegames" directory. They have indeed moved to "playground/games" 11 hours ago.

Other small change: classes "ShowNextPiece" and "PlayerProgress" are now not anymore inheriting the interface "Zoomable" from libkdegames. (Obsolete behavior)