[GSoC status report] Week 13

[Téo Mrnjavac]

Still studying for the exam which is becoming a major unplanned PITA,
but I have also made some important progress on my GSoC project. The
hard work that went into the refactor is starting to pay off, so this
week
- I have made some fixes to Playlist::GroupingProxy to make it depend
only on an AbstractModel below it
- added a Playlist::SearchProxy to decouple searching from filtering
and make next/previous progression through a search work
- untangled Playlist::FilterProxy from Playlist::Model to make it
depend only on an AbstractModel below it
- made the Playlist::Navigators and Playlist::Controller use the
topmost playlist proxy.
All this stuff required me to add quite a few new methods to
AbstractModel and forwarders to ProxyBase, but I'd say that at this
point the AbstractModel interface is almost finalized.

It has been a very eventful week so there's only one major issue left
(and a few minor) before working on the GUI: I need to fix the view so
that operations on a selection of tracks use topmost proxy rows but
reflect on the right rows in the source model. Work on this is already
underway.