Preserve invalid Qt versions when loading a project

Now that we have 2.0 out we can be less strict when checking
project consistency when loading .user files.  The strictness
was necessary due to upgrading non-target projects to target
projects.

 * Allow for invalid Qt versions when loading a .user file that was not
   updated from the pre-target era.
 * Update the UI to show invalid Qt versions as invalid. Remove
   the Invalid version as soon as a valid one is imported.

This does preserve the buildsteps, too.