This commit implements radial gradient as background and adds the option
to settings. It also renames "Color gradient" as "Linear gradient",
keeping Linear gradient default. Internally, it remains unchanged for
compatibility.
Radio gradient is more suitable for CAD, since it gives a more balanced
color distribution across the screen, improving visibility of model and
sketches with a halo-like effect.
Preference Packs are collections of preferences that can be applied en
mass to the user's current setup. Any preference that can be stored in
user.cfg can be stored in a preference pack, and they are designed to be
easy to distribute.
Support is also added for saving a subset of current preferences into a
new preference pack in order to facilitate easy creation of new
"themes", etc.
