This solves an issue in GScrollableWidget where hiding one of the two
scrollbars needs to trigger a relayout since the other one should grow
into the "shared space" in the bottom right corner.
A GWidget can now override custom_layout() which will be called at any
time we would normally delegate work to the GLayout, e.g on resize
or child visibility changes, size policy changes, etc.
Instead of LibGUI and WindowServer building their own copies of the drawing
and graphics code, let's it in a separate LibDraw library.
This avoids building the code twice, and will encourage better separation
of concerns. :^)
