ephy-window: Use a page instead of popover for mobile tabs
As per https://gitlab.gnome.org/Teams/Design/app-mockups/blob/master/web/web.png, on mobile tabs should use a separate page from the main view.
CC: @aplazas
As per https://gitlab.gnome.org/Teams/Design/app-mockups/blob/master/web/web.png, on mobile tabs should use a separate page from the main view.
CC: @aplazas