versioncontrol: Prevent a use-after-free in UpdateItemStatesThread

versioncontrol: Prevent a use-after-free in UpdateItemStatesThread

UpdateItemStatesThread kept a pointer reference to m_plugin whose
lifetime was tied to VersionControlObserver parents.
On application shutdown it could happen the thread is still running when
the Observer is destroyed.

Make the plugin pointer a weak reference, allowing to stop the thread
when the plugin is gone.

BUG: 477425