#6718 Extend PopupDateField range - allow selection of the last days of the

#6718 Extend PopupDateField range - allow selection of the last days of the
preceding month and the first days of the following month without switching
to that month

Changes:

- Internally use Date objects instead of day-of-month integers to track the
  selected and focused date

- Keep track of the currently displayed month separately from the currently
  focused date

- New CSS class for off-month dates: .v-datefield-calendarpanel-day-offmonth

- Off-month days map to negative or past-the-end-of-month integer ids in
  getSubPart functions

- The popup now always shows exactly 6 weeks, so that depending on the month,
  1 to 7 days of the preceding month and 4 to 13 days of the following month
  are shown

svn changeset:22758/svn branch:6.8