mainwindow: Fix the save-as call when closing an unsaved project
mainwindow: Pass correctly the uri of the project which failed loading
project: Cleanup the logic tracking whether all the assets of a project loaded

This actually fixes a bug: When an asset cannot be found and no
replacement is provided, the _projectManagerMissingUriCb method was
disconnecting itself as the missing-uri signal handler and then
_destroyedCb tried the same and failed.