Fix problem with ref counting for instances of FrogrPicture

The data type in the tree model was incorrectly declared as G_TYPE_POINTER,
instead of G_TYPE_OBJECT, causing issues when adding/removing elements
to the model (pictures not reffed/unreffed). Additionally, there were some
missing and extra ref/unref calls that were easily causing unbalanced
situations when doing simple things such as opening the detail dialog,
adding to photosets or even visualizing the tooltips.

https://bugzilla.gnome.org/show_bug.cgi?id=720156
Fix problem with ref counting for instances of FrogrPhotoSet and FrogrGroup

The data type in the tree models was incorrectly declared as G_TYPE_POINTER,
instead of G_TYPE_OBJECT, causing issues when adding/removing elements to the
model (pictures not reffed/unreffed). Additionally, there were some missing
and unref calls that were causing leaks each time a dialog to add a picture
to a photoset (or create a new one) or a group was opened and then closed.

https://bugzilla.gnome.org/show_bug.cgi?id=720156
Fix memory leak in flicksoup: SoupBuffer with file contents was never freed!

Also, moved memory management for the contents of the file loaded to its
right place (the finish callback for the file load operation) and create
the SoupBuffer with SOUP_MEMORY_TEMPORARY instead of SOUP_MEMORY_TAKE.

https://bugzilla.gnome.org/show_bug.cgi?id=720156