Add icon for Not-Sorted-State in columnheader

[VisualAlf]

I'd propose a new Icon for the ColumnHeader, signaling the unsorted state. That should be shown if the column 'CanSort' and the sorting is neither ascending, nor descending.

Reason: a novice user doesn't immediately see if a column is sortable, (if it's not already asc/desc.)
The new icon will symbol that.

Since this feature acts as state indicator as well as hint, I'd make the opacity a bit lower.

The columns are as follows:
C1: CanSort=false, CanFilter=true
C2: CanSort=true, CanFilter=true, State=unsorted
C3: CanSort=true, CanFilter=true, State=Ascending
C4: CanSort=true, CanFilter=true, State=Descending
C5: CanSort=false, CanFilter=false
C6: CanSort=true, CanFilter=false, State=unsorted
C7: CanSort=true, CanFilter=true, State=unsorted, Filtered=true
C8: CanSort=false, CanFilter=true, Filtered=true

What do you think?