TABDEV-3209: adding external objects to content lists
TABDEV-3214: external object icons
TABDEV-3210: added "object" field to external object
TABDEV-3210: string constants
TABDEV-3210: moving file to entity folder
TABDEV-3210: converting url to string before serialization
Merge branch 'TABDEV-3210' into develop
TABDEV-3210: updated project file