Feature to remove unused assets from project

When DS exports a CMake project, it should also sanitize the build environment if needed, thus removing all files that are not referenced by the project. We currently carry along a great deal of assets when we export a (production) project to Creator from Studio.

This should be a toggle in the CMake exporter. Already the CMake exporter knows all the files, now we just should mark the ones that are not used and remove them from the export. 

peter.rohles  provided a simple regexp to start with: Here is a script that returns a list of svg assets of used in your project.  It would have to be altered so that it returns the inverse - assets not used, and then remove those assets from the project.  We would like this for all asset types

grep -r InsertProjectDirectoryHere --include *.qml --regexp \.svg | sed 's/^.*assets./ /'