BorderContainer: unnecessary calls to dojo.marginBox() etc.

[bill]

BorderContainer makes and causes unnecessary calls to dojo.marginBox() etc., slowing down performance and also probably causing the layout glitches in test_Gui.html (#7123).

See the TODO statements in the code for details.

[bill]

(In [14360]) Performance fixes for BorderContainer? (and also possibly fixes layout problems:

• save border box size in _LayoutWidget.resize() and access it in BorderContainer?.layoutChildren(), rather than calling dojo.marginBox() unnecessarily
• modified _LayoutWidget.resize() to take optional second parameter specifying current size. When resize() is used to notify a widget that it's size has changed (rather than being used to set the dimensions), this avoids a call to dojo.marginBox() to query the current size
• make BorderContainer? pass the current size of each child widget to child.resize() so that the child doesn't need to call dojo.marginBox() to query it's own size

Fixes #7138 !strict