Redesign the user interface.

The new theme aims to be flat and minimalistic. Spacing is
applied more generously, unlike colours which are subdued,
mostly gray-blue, with some orange highlights.

Layout changes:

1) Options and Refresh buttons used to be tiny and awkward.
   They have been labelled, enlarged and moved to the top of
   the sidebar.
2) A single button for toggling headlines view wasn't the best solution.
   Mentally, the action is about switching between two different views,
   so two separate, radio-like buttons feel more natural than a checkbox.
3) Similarly, instead of two checkbox-like buttons to toggle filtering
   unread items and bookmarks, there are now three buttons for switching
   between all/unread/bookmarks.
4) Mark View Read has been moved to the right of the view title,
   alongside the other buttons.
Turn the Options button into a dropdown.

The dropdown is a good place to put various important features that
aren't used often enough to be shown in the primary UI.

1) Organize Feeds - opens Library with the Brief's home folder selected.
2) Keyboard Shortcuts (never really fit in the Options window).
3) Options
Custom styling for menu popups.

Now that the rest of the UI is completely non-native, the native styling
of menu popups can be quite jarring on some systems.
Define query constraints for special views in a single place.
Remove the Unread folder and replace it with a Today folder.
Fix and improve the option to exclude certain feeds from global views.

This option used to affect just the Unread folder, which doesn't exist
any more. Let's reword it and make it exclude the feed from both
All Items and Today views. Arguably, that's more in line with the
use case it has been intended to solve, than it used to be before.
Next part of interface redesign.

Adjust spacing in the feed list, remove folder title border,
tweak update progress bar, move bookmark button to the right
side of an item etc.
Tweak item selection highlight.
Add a more convenient and discoverable way to collapse items.
Replace feedview.showOnlyHeadlines with a multi-state integer pref.

This makes it possible to add more view types in the future.
Rename FeedView.headlinesView to a more logical headlinesMode.
Add viewMode column to the feeds table.
Remember view mode separately for each feed.
New design for the sidebar buttons.
Get rid of simple, one-line commands by inlining them.
Separate context menu commands that will be also shown elsewhere.
Add a drop-down button near the feed title.

Some important actions and features are only available through
the feed context menu. A drop-down menu associated with the
title of a feed will make them more discoverable.
Rename various strings, make the nomenclature more consistent.
Do not scroll to the first entry when refreshing a feed view.
New checked state for bookmark item button