Fix possible crash on quit

When quitting, we still have to destroy the Scintilla widget to avoid
any possibility for us to receive signals from it after we destroyed
the associated editor and/or document (used in signal handlers).

I myself don't suffer from the issue, but it is theoretically possible
for Scintilla to emit signals anytime before it is destroyed, so it is
safer like this anyway.  And an user on IRC suffered from crashes on
quit because of this issue, so it seems to actually happen in some
situations.