StatusView: compute font size based on scrollbar size

Compute a font size that just fits the available space, instead of using
an arbitrary scaling. This makes the code adjust to any font and any UI
size.

Select the appropriate font using a binary search, which will need only
a few attempts (I think 6 font sizes will be tried at the default
config)

Change-Id: Ie3b8678678c0d940981f1785418aa8ab354d01c5
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3893
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>