added support for level specific information text messages

added support for level specific information text messages

By placing a text file into "docs/levels/XXX.txt" inside the level set
directory (or directly inside the level set directory just next to the
level file itself), the text message can be shown by pressing the
envelope icon that appears in the bottom left corner of the main menu
(similar to the level set information by the icon on the bottom right,
if available).

The level information text can also be accessed from the info screen.