Implement flexbox (#277)

This implement the flexbox elements, following the HTML one.

Built from them, there is also the following elements:
- `paragraph`
- `paragraphAlignLeft`
- `paragraphAlignRight`
- `paragraphAlignCenter`
- `paragraphAlignJustify`

This is a breaking change.

examples/dom/CMakeLists.txt

@@ -11,6 +11,7 @@ example(gauge)
 example(graph)
 example(gridbox)
 example(hflow)
+example(vflow)
 example(html_like)
 example(package_manager)
 example(paragraph)