Add missing selection mode behavior in parallel coordinates chart

The parallel coordinates chart supports new keyboard shortcuts for different selection modes:

• Press Ctrl while selecting to add to the current selection.

• Press Shift while selecting to substract from the current selection.

• Press Ctrl+Shift while selecting to toggle compared to the current selection.

As GetMouseSelectionMode function is now used by both vtkChartXY and vtkChartParallelCoordinates, it has now been moved to the parent class vtkChart. The static function with the same logic is now called GetSelectionModeFromMouseModifiers. Moreover, vtkChartXY::GetMouseSelectionMode is now deprecated.