Reorganised config dialog

Added a disk image - an entry in ImageChoices which has no filename,
which is displayed as a filled circle. The colour of the circle can be
changed.
The yellow disk svg was removed (no different to the disk option) and
replaced with custom.svg (radial gradient).
Made the calibration grid/craters selectable instead of always being on.
These options default to off and their state does not persist.
Replaced the two image selector buttons with tool buttons.