Add support for column sorting.

Add support for column sorting.

* src/main/java/org/glom/web/client/LayoutListView.java: Change
AsynDataProvider to be an anonymous inner class. Use new
getSortedTableData RPC method when column sort is requested. Set all
columns sortable and add an AsyncHandler to activate sorting in the
AsyncDataProvider.
* src/main/java/org/glom/web/client/OnlineGlomService.java: Add new
method getSortedTableData(). Cleanup other method signatures.
* src/main/java/org/glom/web/client/OnlineGlomServiceAsync.java: Add
new method getSortedTableData(). Cleanup other method signatures.
* src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java:
Implement getSortedTableData() and getTableData() methods by using a
private helper method with the appropriate parameters filled in. Use
user supplied sort clause when supplied, otherwise fall back to
sorting by the primary key. Move destroy() method to be underneath
constructor for readability.  Cleanup comments.