This is an initial attempt at a regression testing framework.

Initially it has some regression tests that come from the work on
improvements to the msg box.

The framework allows the tests themselves to remain extremely simple. They
just need to send a number and potential error message to the outlet.

However, they do need to be added in the main patch. This is because we
will need to run some tests in a certain order. We could number the test
filenames but that's less easy to change later.

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.