Optimize BUrl copy.

BUrl is passed by value in many places, and we should make sure this is
as efficient as possible. There is little point in initializing all the
strings then overwriting them by using the copy constructor, when we can
set them directly.
WebPositive: improve "show bookmark bar" menu item

* Disable the menu item when the folder doesn't exist, as the bar won't
show in that case.
* Use a mark on the item to tell wether the bar is visible, rather than
changing its label
* Simplify the logic for hiding and showing the bar. It is safe to call
_ShowBookmarkBar even if the bar doesn't exist, so no need to check for
it everywhere.

Fixes #11199.