Add content-in-external-file attributes to XSD for interface operation; implemented support for interface operation.
Refactoring : use constants, abstract xml manipulations in utility class.
Merge branch 'store-step-content-in-file' into externalize-interface-requests

* store-step-content-in-file:
  Refactoring : use constants, abstract xml manipulations in utility class.

Conflicts:
	soapui/src/main/java/com/eviware/soapui/impl/support/ContentInExternalFile.java
	soapui/src/main/java/com/eviware/soapui/impl/support/ContentInExternalFileProjectListener.java
Adjustment after merge.