* Introduced guiName to PropertyOwner
* Added requirement that PropertyOwner::identifier may not contain whitespaces
* Changed Name to Identifier in asset and scene files
* Added new PropertyOwner to RenderEngine that owns the ScreenSpaceRenderables
* Moved Name and GuiPath into GUI group
* Added user-facing names to layer groups
* Add implementation of pure URL synchronization from foreign sources
* Added hash of concatenated URLs to the identifier
* Make the identifier optional
* Remove unused code
* Adding ScreenSpaceSpout object to display spout textures in screen space
* Enable the clearing of option properties
* Fix crash in ScreenSpaceImageLocal specification
* Split RenderablePlane into RenderablePlaneImageLocal and RenderablePlaneImageOnline
* Add RenderablePlane Spout
* Rename Texture parameter in ScreenSpaceImageOnline into URL
Closes#518