Added event handlers for gamepadconnected GamepadEvent

[elyseyao]

Added event handlers ongamepadconnected and ongamepaddisconnected to add their corresponding events to lib.dom.d.ts and resolve this issue. The event handler names were taken from MDN.

[saschanaz]

This is not the right way to add them, as the spec does not define ongamepadconnected/ongamepaddisconnected. We need to file a spec issue first. (Edit: Filed w3c/gamepad#141)

Currently the alternative way is to use attributeless-events instead of general events.

[sandersn]

This is widely supported and the spec change hasn't happened yet. Let's take this now and remove the addedTypes override when the spec change gets merged.