Goal: Facilitate clean transport/input/graphics/blink separation for Mus input handling in renderers.

Approach: Create WebWidgetInputHandler(Client) interfaces that deal with input event-related operations. RenderWidgetMusConnection will operate over these interfaces, creating a separation between input and graphics WebWidget operations that (hopefully, down the road) RenderWidget and friends can also exploit to better separate concerns. This effort will also involve writing IPC-agnostic implementors of InputHandler interfaces, which should be easier to test then the current RenderWidget/WebWidget world.