Documents toolview: Remove margins in toolbar

With this patch, the toolbar of the Document's toolview has the same height as the navbar. With KToolBar, this was unfortunately not possible, since it seems KToolbar always adds a top margin, this seems a bug in KToolBar.

Still, with QToolBar, no functionality should be lost.

Before patch:

After patch:

As you can see, in the final screenshot the height of the navbar and the height of the toolbar are identical, which leads to a nice alignment especially in the case of where we only have the navbar visible.

If tabbar + navbar are enabled, this alignment breaks of course again: