Add function to get default type and fix auto-setting prop.type

Hey @lightsofapollo, @KevinGrandon,

I have a few fixes here that change type setting. I noticed there was no function to get the default type without messing around in the design data.

Then I noticed, that setting an  ICAL.Time with isDate=true doesn't automatically change the type to "date", even though we have an icaltype member that would tell us just this.

This patch fixes both issues, I'd appreciate if one of you could take a look.