Add flex_grow and flex_shrink.

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.

examples/component/toggle.cpp

@@ -49,7 +49,7 @@ class MyComponent : public Component {
     });
   }
 
-  bool OnEvent(Event event) {
+  bool OnEvent(Event event) override {
     if (event == Event::Return) {
       on_enter();
       return true;