ISIS-636: guard added to BookmarkServiceDefault

... to guard against NPE for null Bookmark
ISIS-632: further minor tweaks.
ISIS-630: fix parsing of joda LocalDate, and defaults in action prompt

Reworked implementation of JodaLocalDateValueSemanticsProvider, no longer converts to an underlying java.util.Date

Also, ensured that action prompt's args are cleared on cancel, so that defaults for action args are always obtained
if an action prompt dialog is shown, cancelled, then shown again.
ISIS-630: also reimplemented VSP for Joda LocalDateTime

... so it too no longer has any dependencies on the concept of UTC.