This is a preliminary commit for upcoming change where we will use
"bot_creation_policy_values" like approach for many other settings where
dropdown represents the only single setting of integer type.
This commit adds a new setting to the user's notification settings that
will change the behaviour of the unread count in the title bar and
desktop application.
When enabled, the title bar will show the count of unread private messages
and mentions. When disabled, the title bar will act as before, showing
the total number of unread messages.
Fixes#1736.