Allow ... centralized dialog up front

There is a problem confronting applications desiring to use a variety of
APIs such as pointerlock, fullscreen, WebRTC, local storage and so on.

The problem is that each instance of attempting to use such an API leads to
a new "Allow ..." popup the user has to click away.

By some discussion on the www-dom ML the same misery is about to befall
things like translating a Dom Event-Level 4 key code to a users locale.

I would propose to centralize this and make it an up-front dialog
remembered for a site such that:

- The application can trigger the dialog before attempting to use any of
the APIs
- It can get the results back what the user allowed the page to do
- For positive results the page will not have to double-check each APIs
availability/success/failure individually but can assume it's enabled.
- Unless the preference of the page changes in relation to the stored
preference, the dialog (or any popup) for the desired functionality will
not be pestering the user henceforth.