Add a new CalDAV calendar in Roundcube 1.2.1 & All-day events

[HuidaeCho]

These two commits fix a JSON parse error & all-day event shifts.

[jonzl]

I can confirm, that these commits fixed the add new calendar bug!

[Offerel]

With this Patch, you can add a CalDAV Calendar, but if there is more than one Backend defined, you only add CalDAV. You didn't have the choice to what for Backend you want to add the new Calendar.