runtime: remove support for multiple components per unit

[dbartolini]

• [x] Make all comp mgrs accesors accept comp inst ID as opposed to UnitID
• [x] Rename all instances() to instance() and uniform return code in lua_api.cpp
• [x] project: do not generate multiple components of the same type inside a single unit (e.g. in mesh_import())
• [x] Create extra node root to accomodate multiple root nodes
• [x] unit_compiler: check multiple component of the same type inside a .unit
• [ ] Handle re-importing of meshes (how? only allow re-importing (read renderables and update only those still in the .mesh while discarding the others))