Focus improvements

[0HyperCube]

• Focus style updates displays immediately
• The focus_order example always keeps your focus on the buttons
• Add KEYBOARD_NAVIGATABLE ability to allow for focusable elements that are not in the tab order (the equivalent of tabindex="-1" in html)
• Buttons get focused on click (this triggers the focus pseudo class which equivalent behaviour to html - html also has the :focus-visible pseudo class to only show the style when using keyboard navigation which we may also want)

[geom3trik]

This looks great! Thank you for your contribution. Just a couple of very minor changes and then this is good to be merged. I agree that a :focus-visible pseudo-selector should be added next, along with an outline property, which is on my todo list.

[geom3trik]

Related to #55