Refactor|Client|libappfw: WindowSystem split to generic/client specific

WindowSystem was moved to libappfw, with the client-specific portions
remaining as ClientWindowSystem.
Refactor|libappfw|Client: Moved window draw requests to BaseWindow
Refactor|libappfw|Client: Cleanup of the window draw operation
Refactor|libappfw|Client: Basic key/mouse event handling

BaseWindow now handles the default key/mouse event handling for events
originating from the Canvas. The events are passed first to the
WindowSystem and as a fallback to a virtual handler method.