playbackpopover: Limit list length to 10 entries

As specified in the wireframes
https://github.com/gnome-design-team/gnome-mockups/blob/master/
music/wire-playback-buffer.png
3 jobs for wip/feborges/playback-popover in 21 minutes and 49 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Check
failed #7016
allowed to fail
pep8

00:04:53

passed #7015
pyflakes

00:04:57

 
  Build
passed #7017
build

00:11:58

 
Name Stage Failure
failed
pep8 Check
    There are still many devices around that are limited to 80 character
lines; plus, limiting windows to 80 characters makes it possible to have
several windows side-by-side. The default wrapping on such devices looks
ugly. Therefore, please limit all lines to a maximum of 79 characters.
For flowing long blocks of text (docstrings or comments), limiting the
length to 72 characters is recommended.

Reports error E501.
ERROR: Job failed: exit code 1