Support in-line content (images) in XLIFF 1.1 files

Supporting in-line content (images) in XLIFF 1.1 files, as defined by the [standard](http://www.oasis-open.org/committees/xliff/documents/xliff-specification.htm#Struct_InLine), would be helpful when working with External Linguistic Service Providers (ELSP).

It is common to provide contextual information, i.e. images, along with the text labels when outsourcing the translation tasks to an ELSP. Enabling in-line content would leverage the context images that can be added to a set of label keys in Weblate.