Integrate RowTreePlot and ColumnTreePlot classes into a more generic class TreePlot

[RiboRings]

The RowTreePlot and ColumnTreePlot panels share many features with each other, so it would be ideal to create a parent class (for example TreePlot) that contains those similarities, while keeping only the truly different components in the Row and ColumnTreePlot child classes.

Examples on how this could be done are available from original iSEE package, for example DotPlot virtual class, from which RowDotPlot and ColumnDotPlot classes are defined, or the Table class, from which RowTable and ColumnTable classes are defined.