WIP: Fix menu margins for elementary stylesheet

Currently, menu margins are hardcoded to expect Adwaita. This branch moves those margins into the Adwaita stylesheet so that menus look as expected in the elementary stylesheet. Adwaita should look exactly the same before/after, but elementary looks like this:

Before:

After:

Note that in elementary, menu separators should go edge-to-edge

This replaces our current distro patch https://bazaar.launchpad.net/~elementary-os/elementaryos/os-patch-epiphany-browser-bionic/revision/13