abstractions/gnome should allow ~/.cache/thumbnails/fail/gnome-thumbnail-factory/*.png

[Vincas Dargis]

On Ubuntu 17.10 (currently alpha) Gnome file dialog produces this audit entry:

type=AVC msg=audit(1499000321.848:8248): apparmor="DENIED" operation="open" profile="skypeforlinux" name="/home/vincas/.cache/thumbnails/fail/gnome-thumbnail-factory/39e2023d634480a9852aca5e4d7bb600.png" pid=15763 comm="pool" requested_mask="r" denied_mask="r" fsuid=1000 ouid=1000
type=SYSCALL msg=audit(1499000321.848:8248): arch=c000003e syscall=2 per=400000 success=no exit=-13 a0=5c01cf00f00 a1=0 a2=0 a3=1 items=0 ppid=1 pid=15763 auid=4294967295 uid=1000 gid=1000 euid=1000 suid=1000 fsuid=1000 egid=1000 sgid=1000 fsgid=1000 tty=(none) ses=4294967295 comm="pool" exe="/usr/share/skypeforlinux/skypeforlinux" key=(null)
type=PROCTITLE msg=audit(1499000321.848:8248): proctitle=2F7573722F73686172652F736B797065666F726C696E75782F736B797065666F726C696E7578202D2D65786563757465642D66726F6D3D2F686F6D652F76696E636173202D2D7069643D3135373536

(later it asks for 'm' and 'w' also).

Adding:

owner @{HOME}/.cache/thumbnails/fail/gnome-thumbnail-factory/*.png mrw,

into "abstractions/gnome" fixes this problem.

I can provide merge request if this seems reasonable for "abstractions/gnome".