Documentation, legal terms, and credits

• User-visible changes: About Map (credit the MMapper team) and Map Bug menu entries.
• Made it easier to contribute with an improved README and proper license statements.

Map panel in the main window

• The map lives in the main window by default.
• You can detach it into a separate window with Options > Detach Map.
• Up/down exits are drawn.

WIP on milestone 2

display nearby layers with perspective

Closes #7.