I originally put it in FormWidget because CursorTool was clueless about
painting. This patch adds Tool::on_second_paint() which allows all the
tools to hook into the second paint pass.
This patch implements basic rubber-banding. Perhaps this mechanism can
be generalized somehow, but it's not clear to me how that would work
at the moment.
I'll be reconstructing parts of the VisualBuilder application here and
then we can retire VisualBuilder entirely once all the functionality
is available in HackStudio.
