Menus now have a scroll offset (index based, not pixel based) which is
controlled either with the mouse wheel or with the up/down arrow keys.
This finally allows us to browse all of the fonts that @xTibor has made
avilable through his serenity-fontdev project:
https://github.com/xTibor/serenity-fontdev
I'm not completely sure about the up/down arrows. They feel like maybe
they occupy a bit too much vertical space.
Also FIXME: this mechanism probably won't look completely right for
menus that have separators in them.
Fixes#1043.
fb7a885cae