Document changes to imported 3D assets

Document changes to imported 3D assets.

When you import 3D assets, they are now shown in assets view instead of components view (old "My 3D Components"). 

• A placeholder .q3d file is created for each imported asset in <projectcontent>/3d folder
• User can move .q3d files to different content folder if desired
• .q3d file can be dragged to navigator or 3d view to add imported 3d object to scene. Required import is automatically added.
• Doubleclick on .q3d asset triggers "Edit component"
• Context menu has "Edit component" and "Update 3D asset" options (though the latter may be renamed, still under discussion)
• Deleting .q3d file from assets view also removes the imported component and removes any usages from current document.