Allow filtering of compare results by the "?" status

[MauriceSnell]

Currently when doing a directory compare/synchronise, there are 4 filter toggle buttons for the states of equal, not equal, newer on left and newer on right, plus another pair for singles and duplicates.

Could you please add another filter / toggle button for the files shown with status of "?"?

There are two use-cases for this that I know of:

1) When a content compare is underway, (which can take a long time if comparing large volumes of data), it is useful to be able to change the view to include or exclude the files in the queue to be compared. For example, you might want to do operations like delete on files that have already been compared, while the comparison continues.

2) If there is an error during the comparison, the file stays with the "?" unknown comparison status, even after the comparison finishes. In this case, the status=? files are lumped together with equal files, and you can't separate them using the current filter toggles. One way to get this error status is when using DropBox online-only files, with Dropbox offline or disconnected from the folder. Again in this case I need to be able to select equal files separately from unknown status files.