fixed altk_widget_queue_draw()
fixed AltkButton.expose_event()
added the AltkStyleContext class
added altk_style_context_set_state()
added AltkGC.fg and bg
fixed AltkWindow.draw_rectangle()
fixed AltkStyleContext colors
fixed Dialog.expose_event()
code cleanup
added AltkWindowFlags
avoided redraw of input-only windows
handled mouse button down/up events