Feature proposal: a new "flat view" mode and filter by feed title

[ittner]

I am proposing a new view mode for Liferea that shows all feeds in a single level, similar to the reduced view mode but also showing the feeds with no unread items, and a text box to filter feeds by their titles. Idea is show the filter in the new reduced view too, as it seems useful there too, specially for folks with lots of subscriptions (I subscribe to ~460 feeds and already miss this kind of filter).

The "Reduced view" checkbox in the view menu is replaced by a "Normal/Reduced/Flat view" radio buttons (but feedback is welcome for these names!) :

And the new filter box :

I chose to call this box as "Filter" so it is not confused with the usual search feature, but I would also like some feedback on this.

I still wondering if it is a good idea to also filter by URL: it seems very practical (type "youtube" and get all channels, "reddit" and all subreddits, etc.), but it may also be confusing for some users, as the base for the results is not visible in the UI.

I have a working proof-of-concept in https://github.com/ittner/liferea/tree/flatview-and-title-filter-poc -- this code stills need polishing and documentation, but I am already using it for a while and it also seems good enough for discussion and tests among advanced users (i.e. people who install Liferea from source); If this feature is welcome in Liferea, I plan to continue the development and prepare a proper PR for merge.