rewrite of rqt_console

A major rewrite of rqt_console mainly 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 (#116)
* [x] ordering by timestamp with nanosecond accuracy (not as fast as a sort of a numeric value but stable) (#124, #139)
* [x] adding/removing/updating filters is now a *cheap* operation which does not invalidate the complete view anymore (#132)
* [x] improve logic of custom filter to allow partial selection
* [x] some visual improvements to selected filters when not focused
* [x] verify that save/restore is still working (and using the severity keys instead of the labels)
* [x] fix order of filters in menu when adding a new filter
* [x] fix order of items for node/topic filters (partially #184)
* [x] rendering issues on OS X

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. This is completely related to sorting. Without sorting but using the model order the operations are much faster.