rqt_reconfigure: Editor widget re-write, Round II

Fixed a few issues in this round of changes, see commits for details on which. Most changes are fine-tuning and bug fixes; no changes to the experience.

@dirk-thomas you were totally right about the threading issues - I remember having the same issues when I learned about Qt for the first time, which was in C++. I can't get the segfaults to happen any more, but the increased latency in the signal emission compared to the direct function call did increase the jerky-motion that is visible when moving a slider by holding a key.

I understand why the jerkiness is there, but I haven't found a way to mitigate it yet. I know that MOST of the problem is that all of the parameter updates get received twice, as there are two reconfigure clients present: one which was instantiated in the list, and one which is instantiated on connection.

I've looked into using the same client in both places, but it will require more structural changes than I initially thought it would. I'll keep investigating, but I think the changes in this pull are worth merging sooner rather than later. I'm not sure how long the internal re-write will take.

#### Issues Addressed
* #101
* #107

##### Please pull and test this branch, let me know about any bugs in this pull request.