Relax "impossible position" errors in edit mode

[cosinekitty]

When in edit mode, allow more relaxed editing of position until edit mode is exited. Do not allow exiting edit mode until the position is legal.

[cosinekitty]

Also, now clearing the board causes the game to end in a tie, and you are not allowed to start editing the board!

[cosinekitty]

Ideas:

• Disable "Clear the Board" command unless in edit mode.

• When in edit mode, disable all attempts to generate legal moves, check for end of game, etc. Consider entering a deeper function in Win32_gui::ReadMove() instead of adding special case logic everywhere else.

• Do not allow leaving edit mode if the position is impossible.