Incomplete, but promising strategy for the desktop AP

To run the AP locally, install a copy of the assistantPerformer
directory as a chrome extension and load (an external copy
of) localAssistantPerformer.html into chrome.
The web AP works exactly as before: upload all the files
except those used only by the local AP.

Actually, the extension only needs the extension-specific
files plus the scores folder.

The files manifest.json, contentscript.js and the icons in
the directory extensionIcons are only used when the files in
this directory (assistantPerformer) are being used in a
chrome extension, so they never need to be uploaded to
the web. (I may do so anyway, as a form of backup.)
All other files are identical in both local and web AP.

This commit is incomplete. The contentscript code
needs to be completed to retrieve the XML of score pages
from the extension (uses messaging).