Implement item categorization

[derselbst]

This PR adds support to categorize images into sections.

This took longer than expected because I restructured the entire asynchronous model fillup: Previously, there was one huge class SortedImageModel, which took care of initial filling of the model and watching the directory for any changes. This has been split up.

Now, there is an ImageSectionDataContainer which contains the sections and the individual images (or regular files). This container is threadsafe and accessed for reading by the UI thread and for writing by a backgroundThread. The container is populated by a DirectoryWorker, whenever the directory is changed. Additionally, it watches this directory for changes, adds or removes files, and triggers asynchronous decoding for any new files.
The container in turn is a friend of the SortedImageModel. When adding or removing items, the container calls the beginInsert- / beginRemoveRows functions of the model. In order for this to work without potential deadlocks or delays in event processing, the DirectoryWorker and the SortedImageModel are both living in the backgroundThread, so that we can use Qt::DirectConnection.

We cannot set the SortedImageModel directly for a QListView as both would be living in different threads. However, the QListView has set a QSortFilterProxyModel anyway, which are both living in the UI thread. And the latter then has set the SortedImageModel. Not sure if this is supposed to work, but so far it does.

While the model is filled up in the background, one may observe flickering in the QListView (i.e. any already drawn items disappear, appear again, and so on). Not sure how to fix that.

Special thanks to @pmfoss for demonstrating how to implement this kind of item categorization!