Add iOS popover (Resolves #2 Resolves #3)

[angelosilvestre]

Add iOS popover. Resolves #2 Resolves #3

This PR adds IosPopoverMenu and IosToolbar widgets with demos.

[matthew-carroll]

~Can you add a short video that shows the menu in various configurations, as well as show what it looks like to scroll the buttons when they don't fit?~

I see the goldens. No need for a video of the configs. But how about a video that shows what it looks like when the user scrolls the buttons?

[angelosilvestre]

@matthew-carroll

https://user-images.githubusercontent.com/7597082/192404972-af252225-0514-49e0-b6f0-1063dc16974c.mp4

[matthew-carroll]

@angelosilvestre can you add an ability to the demo that allows the user to click and drag around the screen, and cause the arrow to point towards that offset? And then can you post another video showing that the arrow moves as expected for both styles of menu?

[angelosilvestre]

@matthew-carroll Popover menu

https://user-images.githubusercontent.com/7597082/193695908-b9f60a41-7250-47e2-818e-ed24169068fb.mp4

Toolbar

https://user-images.githubusercontent.com/7597082/193695939-dcc7cfe7-a9e0-4aad-beae-99f9bef9b13e.mp4

[angelosilvestre]

It looks like this calculation is still only leaving space for an arrow on one side. Am I missing anything?

@matthew-carroll I think this was the outdated code.

[angelosilvestre]

@matthew-carroll Changed child constraints to respect screen bounds.