Xpra: Ticket #2369: HTML5 client improvements

Following up #2269.

Many new chrome features can be found here: https://chromestatus.com/features

Worth a look:

Custom scheme handlers: the registerProtocolHandler() method: register the html5 client as handler for xpra:// urls. (and xpra+ssl, etc as per #1894)
Wake Lock API: maybe prevent suspend when fullscreen? (if that's not the case already)
https://developer.mozilla.org/en-US/docs/Web/API/Navigator/deviceMemory expose navigator.deviceMemory to the server? (and avoid using video on low power devices)
InputEvent handling for input methods?
Screen.colorDepth: enable 10 bpp? Note: The colorDepth and pixelDepth attributes return the same value for compatibility reasons. and Note: Some non-conforming implementations are known to return 32 instead of 24.
More about devicePixelRatio): #2410
Window Placement ie: one window per monitor in shadow mode (#1801)
websocket.bufferedamount: the number of bytes of application data that have been queued using send() but that, as of the last time the event loop reached step 1, had not yet been transmitted to the network
WebTransport: a mechanism to send many messages as quickly as possibly, possibly out of order, and possibly unreliably from client to server or server to client
Add lowLatency flag to CanvasRenderingContext2DSettings
Keyboard Lock: While in fullscreen, this API allows apps to receive keys that are normally handled by the system or the browser like Cmd/Alt?-Tab, or Esc. Users can escape keyboard lock (and fullscreen) by holding the Esc key for two seconds.