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
Issue 261 Fix For The Continue Button

I've changed how the continue button searches for the latest saved game so that it can search all the directories which are in the saved game directory.

Before this change the continue button code expected there to only be an Autosaves directory and would crash the game if there was other directories in the saved games directory.

New functions
-------------
tree_ctrl.lua:FileTreeNode:
+isDirectory()
+getMostRecentlyModifiedChildFile(file_name_filter):
This new function is now the function responsible for finding the latest saved game for the continue button to load. It is called recursively for all of the directories in the saved games directory.