Allow user custom actions to be independently non-vital

Resolves issue 4424 by allowing User elements to be attributed as non-vital so the whole install isn't failed just to add a user to a group (which can be done after install).
Prevent the current user from being removed

When the test case for issue 4424 uninstalled the MSI package the current user was removed as well.
Merge branch 'patchperf' of git://github.com/heaths/wix3 into develop
Merge branch 'develop' of https://github.com/wixtoolset/wix3 into develop
Merge branch 'issue4424' of git://github.com/heaths/wix3 into develop

Conflicts:
	History.md