Introduce column separator items; change most of ChatView to use qreal

Introduce column separator items; change most of ChatView to use qreal

These items will be used to drag the columns; also they will display a fancy
vertical line at some point (this FR is years old!).

Also changed most (or all?) our geometry-related values to qreal rather than int,
since QGV operates on qreals. Dunno if that is less efficient though; needs evaluation.