use null char to start markdown cell placeholder

[minrk]

'&#x1D106' was added to the front of the placeholder text, to allow 'T' to be typed, but that simply prevented '&' from being typed. A null character should actually be safe until the codemirror bug is worked out.

[fperez]

search for that 1d106 string in the whole code, I'm pretty sure it was in more than one place in the previous PR. If it works, that's fine... Alternatively, if \u can serve as an escape instead of &, it could just be a matter of replacing the escapes.

Though the null has the advantage of being non-printing, which is nice (the musical one shows up). As long as it doesn't introduce other weirdnes..

[minrk]

The '&' is the HTML escape for unicode, but in javascript it's just the ascii string exactly as it appears, so it isn't treated any differently by codemirror than 'T' or '.' was before.

I haven't found any issues, even copying and pasting null-char-including strings into the cell (I think they were stripped out from the clipboard, though).

I just pushed again, after searching for the 1D106, and replacing everything I found.