Update : progress tracking + fullscreen mode

This update contains the following changes.

1.  Added progress tracking. All solved problems are marked with a checkmark in the list of all problems. (The show solution button must not be used.)

 2. Added support for fullscreen mode. You can press ALT+ENTER to switch the game into full screen view or click the "Fullscreen" button in Settings.

3. Reworked descriptions of some problems.

4. Simplified solutions to some problems.

5. Fixed various smaller issues and typos.