Add copy constructors to Drawable and subclasses.

...except for Texture2D and TextLabel (they are removed in another
pending topic) and VolumeGeometry (it is currently unused).

This allows scene plugins to cache a copy of a drawable and
quickly load it into the scene using shallow-copied Core::Arrays
for data members.

This also updates the spheres drawables to uses Core::Array
internally.

Change-Id: Iba55e0ba1411db609c96fdb1be67989de3a2342b
Add the reference overlay axes scene plugin.

This is a feature ported from Avogadro 1.x.

Change-Id: Ib6b3098a88e363708378b2fbd1ebd978043716eb
Merge topic 'reference-axes' into master

5c69f3fd Add the reference overlay axes scene plugin.
af2c5054 Add copy constructors to Drawable and subclasses.