Columns: Removed _fixed_width for indicator columns

- Column indicator formats (depends.indicator, for example), should not set the
  ::_fixed_width flag because then the column does not have the opportunity to
  be suppressed via rc.print.empty.colums=1.

src/columns/ColStart.cpp

@@ -74,8 +74,10 @@ void ColumnStart::measure (Task& task, unsigned int& minimum, unsigned int& maxi
   {
     if (_style == "active")
     {
-      minimum = maximum = utf8_width (context.config.get ("active.indicator"));
-      _fixed_width = true;
+      if (task.has ("start"))
+        minimum = maximum = utf8_width (context.config.get ("active.indicator"));
+      else
+        minimum = maximum = 0;
     }
     else
       ColumnDate::measure (task, minimum, maximum);