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
Fix: Seat count warning update conditions prevented seat messages to appear
Codechange: Deduplicate praise/warn of benches.
Codechange: Remove useless 'else' branch, fixed typos in 2 comments, and
cleaned up the expressions nearby.
Merge pull request #268 from Alberth289346/refactor_hospital

Refactor hospital
Codechange: Deduplicate sitting/standing patients counting code.
Document: Add documentation to Hospital:countPatients()
Change: Simplify vip rating for number of patients standing/sitting ratio
Codechange: Remove lots of useless parentheses.
Merge pull request #270 from Alberth289346/deduplicate_sitting_standing
Refactoring and documenting of world.lua.
In particular World:isTileEmpty was wrongly documented.
Merge pull request #271 from Alberth289346/more_refactor.