Upgrade uikit in the example from uikit 0.8 to uikit 1.0

[myers]

This is not yet complete:

• the metal materials don't look the same as https://pmndrs.github.io/xr/examples/uikit/
• the top right resize handle is moving around as you resize
• I would love to break that big mass of jsx up into components but I'm not going to unless @bbohlender says to do so
• I had to make tweaks to get the "main content" to not space out vertically. This is the most questionable change I made.