client: Fix undescriptive error message when extension module returns nil
client/init.lua: Don't dofile default.lua (it was removed previously)
client: Add Polycode's UI Lua bindings to the build system and Lua environment
client/extensions.lua: Log errors properly when an extension has invalid syntax
extensions/ui: Initial base, with related changes in a lot of places
extensions/graphics3d -> extensions/graphics
CMakeLists.txt: Allow selecting what to build (-DBUILD_SERVER=true/false, -DBUILD_CLIENT=true/false)