Updated Traditional Chinese translation(Hong Kong and Taiwan)
Updated Greek translation
Remove cairo_update_layout() call of a filled cell on draw

Fix for https://bugzilla.gnome.org/show_bug.cgi?id=734890
Do not guess user's desired difficulty level

If the user has never played GNOME Sudoku before, or just has no games
in progress, show the new game screen when starting Sudoku rather than
badly guessing which difficulty level he might want to play.

Usually a game will be saved and launching Sudoku will take you straight
to a puzzle, but if not, one click is not a big deal.

Lastly, it'd be better to present the new game screen than an empty
puzzle, so stop saving those.

https://bugzilla.gnome.org/show_bug.cgi?id=734699
SudokuGenerator: Use multiple threads for puzzle generation

https://bugzilla.gnome.org/show_bug.cgi?id=580056
Make Back button in new game screen insensitive while generating puzzle
Hide the Back button in new game screen at startup
Use GLib.ngettext to show puzzle completion time

https://bugzilla.gnome.org/show_bug.cgi?id=731139
Draw lines better

https://bugzilla.gnome.org/show_bug.cgi?id=734797
Revert to old names for difficulty levels

Also show a warning and fallback to Easy difficulty
whenever a difficulty string cannot be parsed

https://bugzilla.gnome.org/show_bug.cgi?id=734892
Update POTFILES.in and POTFILES.skip
Prepare 3.13.90
Updated Spanish translation
Remove unuseful "else".
Clean properties: remove unused, privatize some, simplify code.
Remove stroke_color.
Refactor public functions used only locally.
Remove unused signals.
Rework functions about selected tile.
Do not initialize popovers and earmarks for fixed cells