Implement UI for browser-level alert/prompt/confirm

[nancyhang]

Hardware: All
Steps to Reproduce:

Go to: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/select
Scroll to Examples.
Click on selection drop down list.

Actual Behavior:
Nothing happens.

Expected Behavior:
A selection list is presented and the user may select an item in the list.

Recommendation:
FxR need to implement UI for the various selection types a web page may present.

[nancyhang]

Linked to #234

[cvan]

Sounds good – thank you! Let's leave issue #234 for implementing the UI for <select>. I've changed the title for alert/prompt/confirm. This week I'll file follow-up issues for the various other fields that need design and engineering work (e.g., some of the <input> fields do not focus at all, which is crucial to fix).

[cvan]

Anyone feel free to disagree, but I vote for this to be a P0 for v1.0. alert/confirm/prompt and Basic-Auth Portal prompts are quite common on web pages. And not supporting them could prevent web pages from being accessed (i.e., password-protected pages), getting user input to properly function (e.g., confirm/prompt), or errors thrown as alerts get swallowed (e.g., all Unity WebGL- or WebVR-exported applications).

[keianhzo]

Those three should be pretty simple to implement +1 for v1.0.

[nancyhang]

This will remain as v1.1 until we get the other high priority P0 completed.

[cvan]

test cases: https://webxr.sh/dialogues (https://webxr.sh/d)

[cvan]

we can use the same UI provided in #234 (comment)