rqt_reconfigure: Re-write widget data flow

rqt_reconfigure is virtually unusable as is, and sends duplicate and unnecessary updates to the server frequently. This patch updates the PyQt widgets' structure to address many of the issues reported with rqt_reconfigure.
Merge pull request #209 from cottsay/rqt_reconfigure-editor-rewrite

rqt_reconfigure: Re-write widget data flow