Partly restore glui custom control classes.
Remove GLUI_Control->type from custom controls.
Fix GLUI_String usage in custom controls.
Fix RGB access in custom controls.
Fix custom control buffering functions.
Update drawing methods in custom controls.

It seems like this GLUI_DrawingSentinal (sic) handles checking whether
to draw and saving state. Hopefully this works correctly on the custom
controls.
Add OO construction API to custom controls.

There's not so much documentation on the newer GLUI's internals, so
hopefully I did this correctly.

Move custom control implementation out of header.
Use new OO construction API for GLUI everywhere.
Fix GLUI_String usage in rest of code.
Fix calls to [gs]et_color.

It seems like the `my_color` variable doesn't exist any more, but based
on its usage in the previous code, the `bkgd_color` is equivalent.
Fix draw_unpressed() call.
Fix activation code.