validation and repairing (closes bitfireAT/ical4android#39, bitfireAT/davx5#8) (#45)

* Drop RRULEs with UNTIL before DTSTART

Note: Validation/repair rules should be better formalized and modularized

* Tests show timezone problems

* validation and repairing (closes bitfireAT/ical4android#39, bitfireAT/davx5#8)

* As discussed in the call

* validation and repairing (closes bitfireAT/ical4android#39, bitfireAT/davx5#8)

* Move validator to validation package

* Minor reformatting

* use time from DTSTART in UNTIL instead of midnight

* merge remote changes

* drop ICalPreprocessor validation rule

* make EventValidator a class with only "repair()" as API and other methods internal

* validate each Event created from a VEvent before saving it to calendar provider, as well as each Event retrieved from calendar provider before creating a VEvent from it

* add tests to cover timezone cases

* stop use of strings and parsing for date building and add a until timezone in test

* [WIP] fix test with timezone offsets

* be more lenient with the result when repairing events where time is cut off

Co-authored-by: Ricki Hirner <hirner@bitfire.at>