Support Dialog.resize(dim) and thus dojox.layout.ResizeHandle.

Note that setting an initial size on a Dialog.domNode via style="..." or indirectly via
class="..." doesn't work, because it's hard to detect when the user wants to explicitly
size the dialog vs. when they want it to size itself naturally based on its content.
If desired we could add a flag to control this behavior.

Fixes #17350.