We should probably return *something*.

I'm sure this isn't correct at all and if there's any situation where radiant:play_music is event'ed we're probably doomed, but eh.
Reinvented rp_spawnstuff.

- Now using the newly introduced start menu API.
- Instead of spawning at the banner, entities are now selected in the main menu and can then be placed in the world.
- It's possible to spawn multiple of the same type (pressing shift)
- Rotating using right mouse button; rotating by 15° is possible by holding shift. Note that these rotations are not exactly well supported and might break stuff. Dragons be here etc.
- The list is sorted after mods (name taken from manifest.json:info.name, icon taken from manifest.json:info.icon), then entity folder, then the entity itself
Woops.
Renaming rp_spawnstuff to rp_spawn_stuff.

(Also, minus one debug thing.)
Rename rp_mergename to rp_merge_names.

Yes, that's official now.