Changed the MenuItem and the Volume Control bar to gain focus when using tab navigation.
Previously, they could not be navigated to via the keyboard.
Fixs #1517.
Tested via manaul cross-browser tests using keyboard navigation (chrome, firefox, safari, and IE 11 ).
reverted the submenu behavior. it now needs to activated before it menuitems become tab targets. The volume control bar now uses the vjs-lock-showing class instead of new class.
Hide the volume bar by setting its content's width to zero.
Reorganized the volume menu button styles and removed some duplicate definitions
Merge branch 'review-rpless-feature/accessible-volume-menu'
Added a line to the changelog for #1519. closes #1519