Corner grabbing area for easier resizing of group-manager window.

[tomaswindsor]

It is currently pain to resize the group manager window. Corner grabbing area (or at least a thicker border) would help.

[nschimme]

I did some testing here. It's MUCH uglier if I increase the border size.

[nschimme]

https://forum.qt.io/topic/60122/increase-floating-qdockwidget-resize-margin-size/3

[nschimme]

https://stackoverflow.com/questions/33392863/gnome-gtk-increase-floating-dock-widget-resize-border-width

[nschimme]

Bug seems related to Gnome