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.
rqt_reconfigure: Update maintainer
rqt_reconfigure: Cache received param changes

rqt_reconfigure: Unify update value naming

Currently, the 'old value' is cached when we send changes (so we don't send an unnecessary update). When the value is changed externally, however, we should update the 'old value' to prevent unnecessary updates here, too.
disable info message when selecting nodes
rqt_reconfigure: Clean up console messages
rqt_reconfigure: Improve checkbox behavior
rqt_reconfigure: Use signals to trigger GUI updates
rqt_reconfigure: Use a single dynamic reconfigure client