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.
Two new elements:
- flex_grow : Expand the element to occupy free space.
- flex_shrink: Minimize the element leave away missing space.
flex = flex_grow | flex_shrink.
Other changes:
- hbox and vbox are now non flexible by default.
- the vtext element has been added to help writting tests.
- Many new tests.
