Improve accessibility of the title of the settings

Before every setting page had the same title and this is causing issues
for screenreaders since they can't differenciate the title of page. Now
a new variable is available for apps to declare the page subtitle.

This new variable is implemented for the settings app and while at it I
added a bit more type hinting to the stuff I touched :)

Signed-off-by: Carl Schwan <carl@carlschwan.eu>