Don't always rebuild on "make" (in the phony "all" target), but
introduce a real file target dist/index.js and its requisites. This gets
rid of the "install-only" target, which is very non-standard and would
not on an unbuilt tree anyway.
Also add a "devel-install" target that does the usual symlinking of the
built source tree into ~/.local/share/cockpit.
Closes#17