Issue #335: Add Multidocument Mode feature (dismine/valentina)

New issue 335: Add Multidocument Mode feature https://bitbucket.org/valentinateam/valentina/issues/335/add-multidocument-mode-feature

Susan Spencer:

Add feature to enable opening multiple documents.

Simple Example: Select one measurement file. Open base garment pattern and also open options pattern for that garment (e.g. collar style) Generate detail and output for both patterns.