[widgets][assetwidget] Implement opening of asset related files

This will happen in the users default application related to
the file type being opened.

Fixes #221
2 jobs for 221_implement_file_opening in 10 minutes and 18 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build_linux_36 #166318
docker-build

00:05:50

passed build_macosx_36 #166319
macosx

00:10:09