fix #29516: note input cursor in drum entry mode

The problem is that the cursor was being moved too early - in Rest::drop(), for a rest that had been created by makeGap() but had not been laid out yet.

I added a call to moveCursor() at the end of Palette::mouseDoubleClickEvent(), only in the case of drum entry mode.  I left the old one where it was as it might be useful in other situations.  Seems harmless.