doap category apps
appdata: Add missing word

https://bugzilla.gnome.org/show_bug.cgi?id=734007
doap: add <programming-language>
Updated Greek translation
Remove SudokuGenerator

It will be replaced by QQwing wrapper
Remove SudokuSolver, SudokuRater and DifficultyRating

- Move DifficultyCategory enum to sudoku-board.vala
- Move float difficulty ranges to SudokuBoard and make them private
Updated POTFILES.in and POTFILES.skip
Add missing semi-colons to PO files
Updated Spanish translation