notification: Don't send file icons to the portal

The notification portal only accept themed icons
and bytes icons, so convert file icons to bytes
before sending notifications to the portal.

This was pointed out in
https://github.com/flatpak/xdg-desktop-portal/issues/317
10 jobs for portal-notification-icon in 20 minutes and 5 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #295851
cross-android_api21_arm64

00:01:46

passed #295852
cross-android_api28_arm64

00:01:46

passed #295853
cross-mingw64

00:06:08

passed #295849
debian-stable-x86_64

00:02:20

passed #295848
fedora-x86_64

00:10:53

passed #295856
freebsd-11
freebsd-11-x86_64

00:03:47

passed #295850
G_DISABLE_ASSERT

00:08:29

passed #295854
win32
msys2-mingw32

00:09:44

passed #295855
win32
vs2017-x64

00:08:52

 
  Coverage
passed #295857
coverage

00:01:49

75.2%