Move and rename Star's property "rad" to Astrobj's "radius"

"Radius" seems more meaningful word and matches SSD keyword.
The native C++ class member should be renamed.
Add capacitor and capacity properties to Warpable for Squirrel

The Alpha Centauri mission starts with a Destroyer with its capacitor
fully charged.
Make TexSphere be creatable by scripts

Also changed property to the new scheme and added texture property.

The old method TexSphere::sqf_get had a nasty bug that it delegates to
sqf_set when the property name does not match.
Experimentally add planets to materialized random solar systems

It's quite a show when binary stars have planets for each star.
Change type of OrbitCS::orbiters to ObservableList<OrbitCS>

Since the structure of CoordSys tree getting dynamic, we can no longer
assume that orbiting celestial bodies will never be deleted.
Luckily, our ObservableList works perfectly fine for this issue.
Prevent drawing duplicate star sprites for materialized stars

Randomly generated (not cached) stars look very different from
materialized stars, so displaying both looks very strange, especially
when the stars are binary.
Paint background of overlay labels for stars and systems

Previously, CoordSys labels (enabled by "@" key in the default
configuration) are painted in white character string on the screen,
but it's hard to read if the viewer is near bright stars.
Painting half-transparent background for those labels helps to recognize
in such cases.