Add visual clue that Plugins menu is scrollable

[sloretz]

(Ignition Citadel 3.3.0) It looks like the Plugins menu is missing the Transform Control plugin, but what is actually happening is the vertical size of the Plugins menu is limited and the plugin menu is scrollable.

It would be helpful if there was a visual cue that the menu was scrollable. Maybe an always shown scrollbar, or a gradient at the bottom if there are more choices?

Here's what each might look like:

[chapulina]

Yeah I've seen people caught by this before, thinking some plugin wasn't available when all they needed to do was scroll.

I think we can just make the scroll bar always visible. The fix would go on ign-gui, I think all that's needed is policy: ScrollBar.AlwaysOn here.

[chapulina]

Done on gazebosim/gz-gui#134