i18n: Make translation works for video editor tabs

Now the display_name & help strings in video editor tabs can be extracted and translated.
Slight tweak to get_text
Merge pull request #5 from edx/ned/tweak-pr-3730

Slight tweak to get_text
Merge pull request #3730 from louyihua/localization-fix-video

i18n: Make translation works for video editor tabs