Wasm Accessibility: Support Voice Over navigation on mobile Safari

Navigation on mobile safari is touch based:

• tap an UI element to move focus to that element.
• swipe left/right to move focus to the next/previous element

Investigate how this works with the a11y DOM underlay approach used by Qt, and if we need to make any modifications to event handling. 

Initial investigation

Focus handling

The user moves focus in one of two ways:

• By swiping left/right or up/down. This moves the screen reader focus back and forwards through the UI elements on the page. Does not generate touch events for the ui elements.
• Tap directly on an UI element. This moves the screen reader focus directly to that UI element. Does not generate touch event for the UI element.

Interacting UI elements

• Activate: Double tap anywhere on screen to active the currently selected element. This works for pushbuttons, checkboxes, etc. The screen reader generates events centered on the focused UI element: mouse and pointer down/up, click. The event target is either the UI element, or the front html element (for pointer events)
• Sliders: Swipe up/down to adjust the slider. It looks like voiceover interacts directly with the slider, at least no pointer events are generated.
• Text fields: Double tap anywhere on screen to open the virtual keyboard. It is unclear what the event flow is here.