Splitters were slightly oversized to work around the fact that we
were ignoring 2px on both sides of them. Now that the whole splitter
can be interacted with, we can lose 1px of fat and look great! :^)
A GUI::Widget can now set an optional content margin (4x0 by default.)
Pixels in the content margin will be ignored for hit testing purposes.
Use this to allow frame-like widgets (like GUI::Frame!) to ignore any
mouse events in the frame area, and instead let those go to parent.
This allows GUI::Splitter to react "sooner" to mouse events that were
previously swallowed by the child widgets instead of ending up in the
splitter. The net effect is that 2 more pixels on each side of a
splitter handle are now interactive and usable for splitting! :^)
We were previously cheating by setting the entire splitter's background
color to the hover highlight color. This looked goofy whenever there
were transparent widgets inside a splitter, since the highlighted color
would shine through when hovering.
This was especially noticeable in SystemMonitor, which now looks much
better. :^)
