po-mode check read only on edit msgstr

[Kevin Ryde]

In po-mode I see that po-edit-msgstr checks the read-only state of the
file when you hit C-c C-c after editing the translation.  It'd be nice
if it checked first, to stop you starting the edit and only finding out
later you shouldn't.

It'd also be nice if there was a po-toggle-read-only bound to C-x C-q,
toggling the secret po-read-only state, equivalent to what the ordinary
toggle-read-only does.