Feedreader improvements

### `Support Atom feeds.` ###

+ Works fine, but I haven't fully tested it. Because I can't find any trackers with Atom feeds with direct torrent links or enclosures.
+ Closes #236

### `Fix: 'Read' state is not saved on exit` ###

+ Can always be reproduced. Just click on articles w/o downloading anything and exit.

### `Support marking articles as 'unread' ` ###

+ Since article read state is correctly saved now I just had to implement this, otherwise random clicking in article list (marking those articles read) will render auto downloader unable to download those items
+ This code supports marking only articles unread (not feeds or folders), which will fire `recheckRssItemsForDownload`
    + This is not entirely true. At first I've made it possible to also mark feeds and folders unread. But this introduced a non-critical bug: marking a feed/folder unread while it has read articles matching a known downloader rule will incorrectly update global and current feed unread counters.
        + Feed counter will be set to maximum number of articles in feed
        + This works fine when nothing is redownloaded from article, otherwise you'll hit the bug
        + Bug has any effect only during current session, on restart (or RSS close/open) counters will become correct
        + Uncomment

            ```c++
            // Will require better synchronization with rss downloader
            // myRSSListMenu.addAction(actionMark_items_unread);
            ```

            in `src/rss/rss_imp.cpp` if you want to test this out
        + Yes, this adds dead code