Add MetadataEditorDialog to test app

Change-Id: I550dc628cd35b2519f7a2b46d411ec4597ac532e
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

Change-Id: I3a3c03f36635afcb1e17ef6595a2a52ea25295fc
Merge pull request #29 from ermshiperete/feature/FixMetadataEditor

Improve MetadataEditor on Linux and fix BetterLabel