WIP! local-display-factory: defer initialization for CanGraphical=no seats

Ray Stroderequested to merge
wip/can-graphical-support into master

During startup a seat may not be ready for a display server yet.

This commit changes GDM to wait until the seat reports that it is CanGraphical capable, before trying to put a login screen on it.

Closes bugzilla-migration/gdm#103

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