This flag is used to find global functions defined without a previous
declaration. Usually it spots accidental globals, but in this case it
was just missing headers.
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.
