Refactor ContentPane to inherit from _Container, and act as a _Container, even when there are multiple independently sized layout widgets as children.
Refactor ContentPane to inherit from _Container, and act as a _Container, even when there are multiple independently sized layout widgets as children.

This means that ContentPane is repsonsible for calling startup() and resize() on it's children in order to get them to initialize and layout correctly.

getChildren() does what you would expect... didn't test addChild()/removeChild() but they should work, although behavior is unclear when ContentPane has deep HTML rather than a simple list of widgets as children.

Fixes #7819 !strict.

git-svn-id: ee6e786acc44c9cca5ee17c71a5576c45c210361@15974 560b804f-0ae3-0310-86f3-f6aa0a117693

redaktor/deliteful