Improve plugin resizing

* Make plugin resizing immediate and continuous (rather than using the default dojo animation)
* Call new plugin method resize() when a plugin container is resized, so plugins have a chance to update themselves
* Layer selector plugin implements resize(), revealing more width as container is resized wider
* Clarify existing layer selector resizing by renaming previous resize() function to onContentSizeChanged() (and add some doc)
Merge pull request #188 from RickMohr/improve-plugin-resizing

Improve plugin resizing