Per bug 1377566, OpenRaster format improvements, and also extending using OpenDocument standard.
includes fix for opacity/visibilty;
enhancements: selected layer; mergedimage; OpenDocument compatibility;
Further OraFormat work including incomplete Properties/Metadata dialog
Merge branch 'master' of https://github.com/horkana/Pinta

Conflicts:
	Pinta.Core/ImageFormats/OraFormat.cs