Primary toolbox: button size is now user-settable

In the future, toggling button size will actually cause new button
images to be loaded.  But at present, image size remains the same, while
the buttons are still resized.

The goal here was to allow for a single column of buttons, with all
buttons still visible, when the smallest button size is used.  This
should allow for maximum viewport area, without needing to hide the
toolbox entirely.

In the (distant?) future, I'd like to allow the user to optionally hide
buttons they don't find useful.  That's a nasty UI project, however, so
no ETA at present.