Tool windows are secondary windows with a smaller title bar. The sit on
the layer above normal windows, and cannot be minimized.
These are intended for complex yet non-modal interactions with the
content of a primary window, such as find/replace windows, property
windows, etc.
This implements simple window shadows around most windows, including
tooltips. Because this method uses a bitmap for the shadow bits,
it is limited to rectangular window frames. For non-rectangular
window frames we'll need to implement a more sophisticated algorithm.
