Improvements and fixes

Weekly exports can now fill the first and last week of the selected period out to all seven days.

• With "Full weeks" turned on, the first and last week of the selected period are filled out to all seven days.
• The preview marks weeks that fall only partly inside the selected range with * and explains the mark.
• Weekly points in the preview chart now read CW12, and daily points show only the day of the month.
• A whole month can now be exported as a single row.
• Choosing the tab character as separator now gives a tab-separated file, and the preview shows values containing quotes or the separator correctly.