When a mouse button is pressed inside a window, put that window into an
automatic mouse tracking state where all mouse events are sent to that
window until all mouse buttons are released.
This might feel even better if it only cared about the mouse buttons you
actually pressed while *inside* the windows to get released, I don't know.
I'll have to use it for a while and see how it's like.
I want to try an MS Office 97 "CoolBar" inspired look for my toolbars.
This is only the painting support, we still need hover events to implement
the actual effect.
This widget is far from finished, but it's off to a good start.
Also added a GResizeEvent and GWidget::resize_event() so that widgets
can react to being resized.
