* Reorganize ContainerBase
- Reduce Container overloads using default arguments
- Extract member function pointers to virtual functions
- Separate classes for Vertical, Horizontal and Tab containers
* Collect unpack from NodeDecorator subclasses
* Reduce redundant expansion for aliases
Allow Container::Vertical and Container::Horizontal to have an
external selector, similar to Container::Tab.
This is useful for implementing a menu of menu.
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>
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.
