Column Sorting

[roovo]

Add the ability to choose how tasks in columns are sorted. Preferebly done via a dropdown menu, as per on the file explorer. For now, the intent it to use only fields currently understood by CardBoard, so:

• file name (and line number in the file)
• task title
• due date
• completion timestamp (only make sense for completed column really)

Priority (as used in Tasks plugin) is an obvious addition to this, but I will need to add support for parsing/uderstanding these first - so will tackle this later (track in #153). Ditto for other date types supported by Tasks (track in #94, #203).

[roovo]

see #15 -> if sorting by due date, then place dated tasks above undated ones.