New Main Menu Button: Continue Game

This new button loads the latest saved game.

I've added English,Russian and Polish strings for it to the language files.

The translations were provided by my Polish and Russian friends.

New strings:
* main menu strings: continue
* tool tip strings: continue
* error message strings: no_games_to_contine
Merge pull request #205 from JASheppard/continue_button

New Main Menu Button: Continue Game