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/component/CMakeLists.txt

@@ -5,6 +5,7 @@ example(checkbox)
 example(checkbox_in_frame)
 example(composition)
 example(dropdown)
+example(flexbox)
 example(gallery)
 example(homescreen)
 example(input)