You can now manipulate the widget selection either by clicking and
dragging the widgets using the cursor tool, or by interacting with
the form widget tree view. :^)
We now use the magical widget registry to factory-construct widgets and
place them into the form.
This will need all kinds of work, but it's nice that the mechanism is
working as intended.
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.
