preview-view: Rearrange the code to allow multiple view widgets

Fixes: https://bugzilla.gnome.org/726505
preview-view, preview-nav-buttons: Sliding transition between photos

Fixes: https://bugzilla.gnome.org/726505
preview-nav-buttons: Restrict keynav within the bounds of the model

We hide the buttons when after reaching one end of the model. We
should also disable keyboard navigation in such cases.

Fallout from edc439c92044b04395e88391d8379f4911eb65bc

Fixes: https://bugzilla.gnome.org/726685
gegl-gtk-view-helper: Use cairo API to calculate stride

Fixes: https://bugzilla.gnome.org/733355
gegl-gtk-view-helper: Fix vertical stripes in the rendered image

Fixes: https://bugzilla.gnome.org/733355
theme: Add CSS snippet for GdMainIconView

Fixes: https://bugzilla.gnome.org/733011
embed: Print a diagnostic if loading the item failed