background: Add CcBackgroundGriloMiner

It fetches the most recent 50 images from every Grilo source that has
a corresponding GOA account. Currently the only such source is Flickr.

https://bugzilla.gnome.org/show_bug.cgi?id=707569
Revert "display: Update for libupower-glib API changes"

This reverts commit 8ba50267bed318cae35a40cd20f3067856e7c93b.
Revert "build: Bump libupower-glib requirement"

This reverts commit e21814150a83ba551a4d1e0fbf0accf9a8bbcb58.
Revert "power: Use the new display device"

This reverts commit 93997f5ee82107e710d1d22b55d6d1137ab97127.
background: Add support for Flickr pictures

Listen to CcBackgroundGriloMiner::media-found and them to
BgPicturesSource.

https://bugzilla.gnome.org/show_bug.cgi?id=707569