RemoteGuiCore: Implement API methods

Implement the API methods specified in the design statement in the RemoteGuiCore:

    @blueprint.route("/tabs/")
    def get_tabs() -> str:

    @blueprint.route("/tab/<tab>")
    def get_tab() -> str:

    @blueprint.route("/tab/<tab>", methods=["PUT"])
    def set_value() -> str:

The methods should work as specified in the raml files and be fully functional.