Add linux desktop file and install (#100)

* extract png icons from the ico

* add desktop file

* add linux install option to cmake

* use 32bit icons

SpaceCadetPinball.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Name=SpaceCadetPinball
+Comment=Decompilation of 3D Pinball for Windows – Space Cadet
+Exec=SpaceCadetPinball
+Icon=SpaceCadetPinball
+Terminal=false
+Type=Application
+Categories=Game;