Put NewStuff.DialogContent's handle into the global toolbar

Without this patch, the dialog's bottom is covered by the drawer's handle, which makes it awkward to get to specifically the close button (but also it just looks very strange). So, using Kirigami's nice, new canContainHandles property on the global toolbar, we put the handle up there instead. It also looks nicer, which is... nice.

Desktop shots of mobile mode (where this problem shows up) - without this patch:

With this patch:

(note these screenshots show an item with categories - without categories it overlaps the close button)