Help browser: Move the index construction into pdgui.js, so that it is only done once during startup.

Help browser: Construct the help index from all of lib/pd-l2ork, to make the keyword search more useful.

Fix up the install/uninstall targets to properly handle the case that the build directory doesn't exist.

Add convenience targets to the toplevel Makefile which install additional abstractions and externals after a light build. E.g.: make light pdlua_ext