Use independent scrollbars for canvas and palette

When using the form-designer, there are two panes (the canvas and palette). Currently, when expanding vertically, both panes alter the height of the dialog content, and this becomes awkward if both panes include long lists. For example, suppose the user wants to drag an item from the top of the palette to the bottom of the canvas – the user will need to simultaneously drag-and-scroll (which feels weird). It's more natural to let the user scroll the canvas, then scroll the palette, then drag-drop.