Rather than using order to specify item precedence, we now construct
a set of menu items for each element traversing upward from the target.
When merging items for a given element, we pass the specificity to the
merge function, which uses it to decide whether or not to clobber
existing items. When assembling the overall menu, we don’t ever clobber
to ensure that items added for elements closer to the target always win
over items matching further up the tree.
Previously I used CSS specificity to order the most specific / recently
added menu items for a given element *first* when building up the
context menu. When a duplicate label was found for a given menu I would
refrain from inserting it. Now instead I order things the opposite way.
The most specific / recently added items come later and items with the
same label are clobbered by later items.
