Improve tool tips for to-dos

There are several problems with tool tips for recurring multi-day to-dos:

• The due date is wrong.
• The start date is just a date, without a time.
• The to-do is displayed in the agenda view on the start date.
• The agenda view passes the start date down to dateRangeText(), which expects the due date.
• dateRangeText() calls getNextDateTime() expecting a due date, but getNextDateTime() returns start dates.

This patch adjusts dateRangeText() to accept any date between the to-do's
start and end dates, adjusts the tool tip contents, and incidentally fixes
BUG: 360730