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. Oh and it's also not safe.
Write some NEWS for 3.14.1
Updated Latvian translation
Updated Italian translation
Prepare 3.14.1
Updated Norwegian bokmål translation.
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
Prepare 3.14.2
Finnish translation update