Add forceWidth config param to dojox.form._HasDropDown

[msorensen]

Setup: _HasDropDown.dropDown is a widget that grows to fit the available space (ex. unconstrained BorderContainer?).

Symptom: The first time the openDropDown is called, the dropDown is correctly resized to _HasDropDown.domNode.offsetWidth. However, the second time openDropDown is called, the dropDown is resized to be as wide as the page.

Cause: During the second openDropDown, _HasDropDown trys to recalculate the natural width of the dropDown by removing the sizing constraints and calling marginBox. Once the sizing constraints are removed, the domNode of the BorderContainer? grows to the page width.

Proposed Solution: Add a config parameter named "forceWidth" to _HasDropDown that would resize the dropDown to match the width of _hasDropDown.domNode.

[msorensen]

Proposed patch

[Nathan Toone]

(In [19072]) Fixes #9589 - add in a forceWidth option for subclasses of _HasDropDown