menu: refactoring (close #363)

  * get rid of `menu__group_has-title` modifier
  * menu-item_checked: use `background` instead of `::before`
menu-item: theme inherit (bh templates)
menu: TODOs + examples
menu: links must inherit `disabled` mod
Merge pull request #364 from bem/issues/363@v2

menu: refactoring (close #363)
button: separate `pressed` and `checked` mods (close #371)
button: get rid of `check`/`uncheck` events in favour of mod change events (close #357)
Merge pull request #372 from bem/issues/371@v2

button: remake `checked` mod
button: pressed && checked state (close #373)
Merge pull request #374 from bem/issues/373@v2

button: pressed && checked state (close #373)
WIP
wip
wip
WIP
wip
_checked
focus
properly change _inPopupPointerPress flag
WIP
remove redundant code