[nemo-qml-plugin-calendar] Only check notebook validity at calendar l…

[dcaliste]

…evel.

There is no need to check for runtime-ness of
the notebook here, it is done at mKCal level
when saving an incidence already.

It is harmfull to check for readonly-ness of
the notebook though, since the read-only
status is only a hint. One should be able to
save modifications in a read-only notebook
when these modifications make sense (like
setting a reminder in the birthday calendar).

It is required to check that the notebook
exists in the calendar, since this check is
not done in mKCal at DB level.