Sort the UI examples into sub-dirs

[ickshonpe]

What problem does this solve or what need does it fill?

There are a lot of UI examples now, which makes it harder for users to find specific examples relevant to their work.

What solution would you like?

Sort the UI examples into categories and move them into sub directories.

We already have a "navigation" sub dir.

For others, I'd suggest ones like:

• "text" for text and font specific examples
• "layout" for the grid and flex layout examples
• "images" for examples using images and texture atlases with ImageNode
• "scroll_and_overflow" for the scrolling, scrollbars, and clipping examples

and so on.

Some larger examples that span multiple categories could remain in the parent ui dir.