Make it possible to navigate with the keyboard in the webxr glwindow view

[paulrouget]

The glwindow is not interactive. With this PR we can use the arrow key to look up, down, right and left.

This will make debugging a lot easier.

---

This change is 

[paulrouget]

This was not as easy as I expected. We need to process events for all windows, but the glutin app never had any reference to the xr' glwindow.

Managing multiple windows will be necessary in the future, so this work is not useless, but it's not complete either (see inline comment about multiple windows vs. multiple browser).

[asajeffrey]

I guess there's a question about which bits of logic should live in servo and which in the glwindow. If we want to put everything in servo this looks fine to me. We'll need most of it at some point when we handle multliple windows properly. You can r=me.

[paulrouget]

@bors-servo try=wpt

[bors-servo]

⌛ Trying commit 3711ea9 with merge 81af4f2...

[bors-servo]

💔 Test failed - linux-rel-css

[jdm]

servo/ports/glutin/headless_window.rs

Lines 135 to 137 in 3711ea9

	fn id(&self) -> Option<glutin::WindowId> {
	None
	}

[paulrouget]

@bors-servo try=wpt

[bors-servo]

⌛ Trying commit e1bc980 with merge 617f418...

[bors-servo]

☀️ Test successful - linux-rel-css, linux-rel-wpt, status-taskcluster
State: approved= try=True

[paulrouget]

@bors-servo r=asajeffrey

[bors-servo]

📌 Commit e1bc980 has been approved by asajeffrey

[bors-servo]

⌛ Testing commit e1bc980 with merge 61bf3ac...

[bors-servo]

💔 Test failed - status-taskcluster

[CYBAI]

Failure in magicleap build:

error[E0053]: method `wait_for_animation_frame` has an incompatible type for trait
   --> /Users/worker/.cargo/git/checkouts/webxr-702fa5244eec1e69/13a57d7/webxr/magicleap/mod.rs:383:5
    |
383 |     fn wait_for_animation_frame(&mut self) -> Frame {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected enum `std::option::Option`, found struct `webxr_api::frame::Frame`
    |
    = note: expected type `fn(&mut magicleap::MagicLeapDevice) -> std::option::Option<webxr_api::frame::Frame>`
               found type `fn(&mut magicleap::MagicLeapDevice) -> webxr_api::frame::Frame`

[paulrouget]

servo/webxr#64

[paulrouget]

@bors-servo r=asajeffrey

[bors-servo]

📌 Commit e65f1c9 has been approved by asajeffrey

[bors-servo]

⌛ Testing commit e65f1c9 with merge ad1d003...

[bors-servo]

☀️ Test successful - linux-rel-css, linux-rel-wpt, status-taskcluster
Approved by: asajeffrey
Pushing ad1d003 to master...