Another big commit, the Kigo GUI moved from a per-screen based approach (with a GameScreen, SetupScreen, ErrorScreen) to a dockwidget based approach with several small widgets which are shown/hidden depending on the game state.

This way Kigo becomes a lot more customizable and expandable in the future.
It also concentrates all GUI-related code in the MainWindow class while the other GUI classes just implement additional logic (like showing SGF file information or a move history list).

The undo/redo support is now more feature-complete with a little dockwindow showing the move list.
I also added a little visual hint which is shown when the user is about to place his next stone.

Before that it wasn't always clear where a stone will be placed when the mouse is clicked, this should be more obvious now.
Last but not least the usual code refactorings and cleanups...