Improve MetadataEditor on Linux and fix BetterLabel

- don't display label disabled
- move BetterLinkLabel to separate file
- fix problem where link label didn't show when it got set to
  visible after having been created as invisible
- fix size of MetadataEditorControl
- align location of "Additional Requests" and "Not Enforcable"
- put buttons in docked panel so that they completely show on Linux