ProjectManager: Warn when projects have different config_version

When opening projects for edition through the project manager, the
following checks are now done:

1. If the config_version is lower than the one used by the current
   engine version, users are asked if they want to convert to the new
   format or abort editing. Fixes #20626.
2. If the config_version is higher than the expected one (project
   from a more recent and incompatible engine version), projects are
   grayed out and can't be edited. Fixes #18758.

When editing from the command line, the behaviour is unchanged:
projects in situation (1) are automatically converted, while projects
in situation (2) show an error message (made more explicit).

The "Run" option from the project manager was not changed, so it will
still run (1) projects without converting them, and fail running (2)
projects.

Co-authored-by: groud <gilles.roudiere@gmail.com>

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