Checkbox inputs

[cfraz89]

Adds support for input type="checkbox", as requested in #94 . Makes checkboxes send oninput event when clicked. If event isn't bound (there's no data-dioxus-id), they will behave as an unmanaged input, and handle checked state internally. If it is, they will send an oninput event with a FormData argument.

Adds a 'form' example to demonstrate.

Caveats:

• They should be drawing their colors using the css property 'accent-color', but I couldnt figure out how to look that up, so they're using just 'color' for now.
• There's something up with the vertical alignment when rendering labels with the text 'inline'. Check the second checkbox in the example, or try removing the 'inline-block' from label styling in the css. In chrome and ff, the labels render the same vertically whether inline or inline-block.
• The FormEvent has valid: false, as its calculated as 'true' if value is empty? However I've set value to the value of the toggled element. Not sure if this is correct
• Checking a checkbox send 'onclick' and 'oninput' events. Not sure if it should send 'onchanged' too? Didnt see any mention of that event in dioxus docs.

[nicoburns]

This PR seems to be breaking button layout on the google example.

main branch:

This PR: