autohide bm and main toolbars: Fix transition and remove separate transform from .urlbar
Bug 2022527 removed a hack on Firefox side that made :popover-open always match.
That in turn caused us to need separate transform for .urlbar elements.
With that hack removed, it seems as if the layout engine optimizes something
and causes the toolbox, which is transformed 90deg, to be transformed to visible
behind web-content, before the layout notices that the toolbox should be at front.
The separate urlbar transform we do now causes its transition to not be at
sync with the toolbox.
This patch changes the toolbox transform to 89.9deg which is essentially invisible
but the layout considers it visible and the animation stays smooth.
The patch also constrains the .urlbar rules to only match when :popover-open matches
so we don't apply these rules unnecessarily anymore.
Additionally, this adds --toolbar-background-image to be received from theme so toolbars
can show if the theme specifies it.