Fixes for the due date picker

[wsdfhjxc]

This pull request fixes the following issues with the due date picker:

• when opening the date picker, the initial selection doesn't match the taks's due date
• clicking the Clear button doesn't remove the due date from the text input field (reported in Clearing the due date no longer works #32)
• sometimes a wrong or missing date is displayed in the dueDatePickerInput if the user clicks outside it
• there is no indication of the current day in the calendar, and in combination with the first issue, it's quite confusing

Also, the current day is indicated in a cell with a light green background. It's not too distracting from the actual selection.

[ransome1]

@wsdfhjxc I checked it locally and again it looks great. Thanks a lot for contributing these improvements.

• when opening the date picker, the initial selection doesn't match the taks's due date
  I just checked the last build (0.8.5) and could not really reproduce this issue
• clicking the Clear button doesn't remove the due date from the text input field (reported in Clearing the due date no longer works #32)
  Great, works fine!
• sometimes a wrong or missing date is displayed in the dueDatePickerInput if the user clicks outside it
  Works!
• there is no indication of the current day in the calendar, and in combination with the first issue, it's quite confusing
  Would you mind if I repaint it to grey? The thing with green is that it is used for contexts too and I would like to avoid any misunderstandings.

[wsdfhjxc]

I just checked the last build (0.8.5) and could not really reproduce this issue

Yeah, I should have preceded that with "sometimes", as it's not always reproducible.

Would you mind if I repaint it to grey? The thing with green is that it is used for contexts too [...]

I'm fine with whatever color that is distinctable from the current selection color and the highlighted cell color.