[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 Job ID Name Coverage
  Build
passed #166318
docker-build
build_linux_36

00:05:50

passed #166319
macosx
build_macosx_36

00:10:09