Add client-side events to allow developers to integrate external systems into Cmdr

[Rythian2277]

I propose adding the following event to the client (with a framework to add more in future):

• Cmdr.Toggled: RBXScriptSignal -> (visible: boolean)

This event would allow for developers to let their game systems, especially their UI, behave appropriately with the Cmdr client.

Example of implementation (but can 100% be done better)

master...Rythian2277:Cmdr:master

[autonordev]

FWIW this could be currently done by pulling PlayerGui.Cmdr (I think there's also a member within the API to get the screengui?) and adding an event listener to the window's Visible property

I do agree in principle that we need to make it easier for developers to integrate external systems and 'plug into' Cmdr, especially on the client. Currently a lot of stuff relies on private members (which aren't covered by our API contract, so could be broken in patches) or are non-determinate (like PlayerGui.Cmdr)