Create and initialize a new EdMode-level ToolsContext derived from the ModeManager ToolsContext. The EdMode ToolsContext does not have it's own InputRouter, it shares the InputRouter with the ModeManager ToolsContext. The ModeManager ToolsContext keeps track of these derived ToolsContext's and automaticallyTick()'s them/etc. When the child ToolsContext is shut down, OnChildEdModeToolsContextShutdown() must be called to clean up