Submenu items not clickable when option "expand the menu in full width below the menu hamburger icon." is checked

[ghost]

Reported : https://wordpress.org/support/topic/mobile-menu-doesnt-work-when-nimble-builder-is-activated/#post-13954336

Reason : this is because NB sets attributes data-sek-is-mobile-menu only when the option "expand the menu in full width below the menu hamburger icon." is checked.
And the click is prevented based on the check on isMobileMenu which is based on this not reliable attribute.

Solution : improve reliability of the data-sek-is-mobile-menu attribute => set the value based on actual width of the device, compared to the current breakpoint. In the future the mobile menu breakpoint will be customizable ( #779 )