Assign materials to segments and shapes

[Henry00IS]

Have a list of materials in the project. The user can assign materials to individual segments as well as the entire shape. The segments will be shown with a different color to indicate the material slot used. Subtractive shapes can be represented using dotted lines where additive shapes will have filled lines as usual. The materials assigned to the segments are used for the extruded sides of the mesh and materials assigned to the shapes themselves are used for the front and back faces of the mesh.

This will make it more user-friendly to change and extrude the same brush later in RealtimeCSG, as this way at least the materials are kept between updates. It also makes the regular meshes more useful. Requires #5.

[Henry00IS]

Implemented a much better way since 841ccde with the 3D material editor.