Refactor code for strict unread message display

Before, when you choose the show unread message option and the hide categories and feeds options, the categories and feeds weren't hidden. Now it is working properly.
Before, the unread filter icon was not selected with the previous way of using the constants. Now, the filter is highlighted when selected in strict mode.

See #619
It is maybe better to disable this option if "Show all articles" is selected,  isn't it?