SudokuPrinter: store a list instead of an array
generator: use a thread pool

The current code blocks the UI thread briefly if the generator threads
do not finish in order.
Remove extra cast
Updated Ukrainian translation
Fix a whitespace mistake
Updated Latvian translation
Updated Italian translation
Add missing warning comment
Updated Spanish translation
Code cleanup as per gnome coding conventions

Code convention fixes

https://bugzilla.gnome.org/show_bug.cgi?id=738930
Require Gtk+ 3.15.0.
Load CSS from gresource.
Larger print-multiple dialog.
Prepare 3.15.1
Update POTFILES.in and POTFILES.skip
Change Finnish translation team web page to l10n.gnome.org
Correctly check XDG_CURRENT_DESKTOP environment variable

"The XDG_CURRENT_DESKTOP environment variable can contain a list of
desktop names. The existing code assumed it could only contain one
name."

Solution by Robert Ancell

https://bugzilla.gnome.org/show_bug.cgi?id=739592
Makes the highlighter optional

The highlighting of the row, column and square that contain
the selected cell was always on.

https://bugzilla.gnome.org/show_bug.cgi?id=734724
Reorder and reword a couple menu items
Updated Spanish translation