#51 made all panels in AdditionalTagsPanel opaque
 #51 select or create of contacts now possible

In MetadataEditor, the textbox which shows a contact was replaced
with a combobox. Due to this
the user now can select another contact. The functionality of the button
was changed in that way, that now it can only create a new contact.
 #51 select or create contact in ResourceEditor now possible

A combobox to select a contact and a button to create a new contact were
added to ResourceEditor. The previously used ContactEditor in that tab
pane is now a ContactRenderer, which shows the selected contact as
preview.
Merge branch 'feature/51-select-or-create-contact' into dev