background: Prevent monitor_pictures_model() running multiple times

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Re-add drag'n'drop support

Inside the chooser dialogue itself.

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Allow dropping colours as well

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Remember added colours

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Split up creation of loading icon

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Make placeholder the same size as thumbnails

Otherwise they look tiny and compacted.

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Ensure screenshots are removed from the list

Even the ones that weren't detected as PNG in the first place.
background: Copy all the manually added backgrounds

Except the ones already in the Pictures directory

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Create a placeholder for manually added items

When adding things that might be screenshots, always create a
placeholder if the image was added manually.

https://bugzilla.gnome.org/show_bug.cgi?id=689351
background: Fix semi-random placement style when selecting "Picture"

When selecting a background from the Pictures source, make sure that
the placement is reset from whatever it was set to before, otherwise
it will make use of the previous one, leading to tiled wallpapers if a
colour was selected before, for example.

https://bugzilla.gnome.org/show_bug.cgi?id=703057
shell: Use correct window icon

The window icon should match the icon specified in the desktop file.

https://bugzilla.gnome.org/show_bug.cgi?id=724937