runtime: remove support for multiple components per unit

[dbartolini]

• Make all comp mgrs accesors accept comp inst ID as opposed to UnitID
• Rename all *instances() to *instance() and uniform return code in lua_api.cpp
• project: do not generate multiple components of the same type inside a single unit (e.g. in mesh_import())
• Create extra node root to accomodate multiple root nodes
• 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))