[GSoC] Comments in Sidebar Deck: Weekly Update

[Mohit Marathe]

Hello everyone,

In the last week, I implemented filter comments by author and date functionality.
I used `SvtCalendarBox` for the date picker, which is basically `GtkCalendar`.
There is one problem though, I don't know of any way to de-select or reset the date
selected. So I decided to make a "Reset" button for it. I tried to set the date in 
the date picker to `Date(Date::EMPTY)` and added some handler to treat this date as
if no date is selected. But it didn't work.
Should I use something like `gtk_calendar_unmark_day` or `gtk_calendar_clear_marks`?

Thanks,
Mohit