Add labels to the device dropdowns in the MIDI setup so that the order in which devices are listed is obvious.

Help browser: Cosmetic changes in the GUI prefs, add a warning about the impact of help browser settings on startup times.

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.