This brings msg box back to the same performance it had before $0 expansion
was added as a feature to message boxes.

The first implementation relied on calling canvas_setcurrent and
canvas_unsetcurrent in the message box methods. This added a malloc/free
for each message box method call.

Instead this implementation caches the msg box glist in the t_messresponder.
That is available as the target parameter in m_binbuf. Unfortunately, we
have to add a parameter to binbuf_realizedollsym. Since that's a public
function we have to break out the new param into binbuf_dorealizedollsym.

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.