properties-dialog: Use a more generous title entry box size

It is really hard to edit a label in a very small box.

Original patch from William Jon McCann for gnome-documents.
properties-dialog: Make rows the same height

Original patch from William Jon McCann for gnome-documents.
properties-dialog: Use enumerated names instead of literal values

Original patch from William Jon McCann for gnome-documents.
properties-dialog: Disambiguate "Type" for translation

This doesn't really affect us at the moment because we have only
instance of this string. However, it is good to guard ourselves from a
future mishap.

Original patch from Cosimo Cecchi for gnome-documents.
properties-dialog: Disambiguate "Title" and "Author" for translation

This doesn't really affect us at the moment because we have only
instance of these strings. However, it is good to guard ourselves from
a future mishap.

Original patch from Cosimo Cecchi for gnome-documents.
properties-dialog: Add some requested translator comments

Original patch from Cosimo Cecchi for gnome-documents.
base-item: Add a pure virtual method for getting the source name
properties-dialog: Show the source for Flickr items
Updated POTFILES.in
Updated Czech translation
base-item: Refactor opening in external application

We have been using gtk_show_uri to open the URI till now. However this
doesn't work for Flickr because the URI points to a JPEG file, and not
the Flickr page for the particular photo. A virtual method with a
default implementation was added so that we can tweak the behaviour
for Flickr.
flickr-item: Set "Flickr" as the default application

Open the actual Flickr page instead of just the raw JPEG content.
flickr-item: Remove redundant headers
Updated Spanish translation
Replace GdStack with GtkStack
Replace GdHeaderBar with GtkHeaderBar
Add missing whitespace
Updated Brazilian Portuguese translation
local-item: Simplify conversion of URI to local path
base-item: Let the download vfunc be publicly available

This will be needed when pushing remote items to a DMR.