Nested tab container and title pane bug

[Katie Vance]

Steps to reproduce:

1. Load dijit/tests/layout/test_TabContainerTitlePane.html
2. that Tab 3 and Tab 4 (nested inside the titlepane) are rendering content.
3. Minimize the title pane
4. Click on Tab 2
5. Click back on Tab1
6. Maximize the title pane

Notice that Tab 3 and Tab 4 are no longer rendering their content.

[bill]

Good catch. When "tab 1" is selected TabContainer calls TitlePane.resize() (which is really ContentPane.resize()), and even though the TitlePane is closed ContentPane.resize() tries to resize() it's children.

Besides that bug, there's another problem that opening the TitlePane doesn't resize() the children, even though this may be their first chance to layout. Changing the test case to make the TitlePane initially closed shows this problem.

[bill]

(In [23276]) Fix problem in layout widget --> TitlePane --> layout widget hierarchy where the nested layout widget's resize() is called while the TitlePane is closed, rather than when it is opened. Fixes #11816 !strict.

[bill]

(In [23346]) Get ExpandoPane working again after [23276] (refs #11816) and [23340] (refs #12078), !strict.

Reintroduced separate ContentPane._layoutChildren() method since ExpandoPane was referencing it and other classes might be theoretically referencing it too.