rewrite of rqt_console

A major rewrite of rqt_console to overcome performance problems of the previous implementation.

* [x] the data storage has been redesigned which does not require accessing information via the `data()` function of the item model
* [x] adding and removing filters is now a *cheap* operation which does not invalidate the view anymore
* [x] improve logic of custom filter to allow partial selection
* [x] some visual improvements to selected filters when not focused
* [ ] verify that save/restore is still working (and using the severity keys instead of the labels)
* [ ] fix order of filters in menu when adding a new filter
* [ ] fix order of items for node/topic filters

The only operation which still takes a long time is when the view needs to add a large number of rows which has not been there before. I currently don't see a way to solve that with Qt.