Revert to type-specific retrieval of date/times from iCal: Bug #733319

To prevent having a big switch-case table pulling out DATE/DATE-TIME
values from properties, some time back changed to getting the property
value, seeing if it was a date or date-time, and fetching that.  Nice
and generic.

However, since then there's been a number of reports of well-formed
timed-based properties (notably DTSTAMP) as being of type DURATION or
iCal's fall-back "X-VALUE", meaning it wasn't transformed into the
data structure we need.  Looking through Evolution and EDS' code, it
appears they don't use a generic access method, and I worry this is
broken under certain use cases.

So, this reverts back to the old way of looking up DATE/DATE-TIME
directly depending on the property type.  Hopefully this will solve
the issue some people are seeing (as I cannot reproduce it here).