Add Scrollbar for plugins widget

[mauxion2]

Add Scrollbar for plugins widget area.

With multiple plugins widgets main window is vertically stretched beyond screen area making update button inaccessible , (I'm using Bioscan, Inaraprogress, NavRoute, Pioneer) adding scrollbar to widget area would improve usability with multiple plugin widgets and prevented stretching of window beyond bottom of the screen.

[Rixxan]

Related to #1293. Marking as a 6.0 as I don't know if this will break anything of how our custom ttk.nb works, but may be able to be done earlier.