Refactor video volume control.
i18n: More extractable & localizable strings in xmodules
Merge pull request #3678 from edx/anton/refactor-volume-control

Video: refactor volume control.
Merge pull request #3878 from louyihua/master

i18n: More extractable & localizable strings in xmodules
i18n: Make XBlock buttons & components have localizable display names

1. Current code uses type name as the display names for XBlock buttons, so the name of these five buttons can't be localized. This fix contains code that adds a 'display_name' field for the buttons, and makes their names localizable.
2. The display names of components in advanced, html and problem types can be localized now.