Add an /import command line option

Provide a command line syntax for importing a game installation into the launcher. This should require the game manifest file location as an argument, with the manifest required to be placed in the games install directory.

This ensures future proofing against when the launcher wants to keep track of installed manifests. In the initial implementation it should just pull the game ID and version from the manifest and put them into its data store.

This will allow standalone installers to integrate with the launcher (which in turn should install or update the launcher if it's not already installed - this should be a feature of the launcher installer)