Add support for managing variants

[bugzilla-to-github]

This issue was created automatically by a script.

Bug 1453324

Bug Reporter: @mathjazz CC: @flodolo, @Pike, @rimas-kudelis, @zbraniecki Depends on: Bug 1493983, Bug 1465121

Rich UI should support:

• adding and removing variants
• editing variant keys
• changing the default variant

[bugzilla-to-github]

Comment Author: @mathjazz

*** Bug #1383048 has been marked as a duplicate of this bug. ***

[bugzilla-to-github]

Comment Author: @mathjazz

*** Bug #1493886 has been marked as a duplicate of this bug. ***

[bugzilla-to-github]

Comment Author: @mathjazz

*** Bug #1493886 has been marked as a duplicate of this bug. ***