Enhance status bar to show length of selection (cols if within a single
line, line(s) or partial lines if selection spans more than one).

Hoist out \t converion logic from EditorgetCursorPos(). Add unit tests for
it.
Fix inconsistency in how pluralizable words are shown in the status bar -
never use the indefinite "(s)" suffix anymore.
Merge pull request #4579 from adobe/pflynn/selection-info

Enhance status bar to show length of selection