Added semi-workaround method for network thread shutdown:

Currently closing game while network thread is waiting for something is
very bug-prone, since network thread may resume during shutdown and
access partially destroyed client state.

Now if exit has been requested, the very first step would be semi-
graceful shutdown of network thread (via exception throwing). This may
in theory skip some cleanup in non-RAII code, but since game is shutting
down this does not matters much.

This logic applies to:
- shutting down while network thread is waiting for dialogs
- shuttind down while network thread waiting for animations in combat

docs/players/Cheat_Codes.md

@@ -142,5 +142,4 @@ Below a list of supported commands, with their arguments wrapped in `<>`
 `activate <0/1/2>` - activate game windows (no current use, apparently broken long ago)  
 `redraw` - force full graphical redraw  
 `screen` - show value of screenBuf variable, which prints "screen" when adventure map has current focus, "screen2" otherwise, and dumps values of both screen surfaces to .bmp files  
-`not dialog` - set the state indicating if dialog box is active to "no"  
 `tell hs <hero ID> <artifact slot ID>` - write what artifact is present on artifact slot with specified ID for hero with specified ID. (must be called during gameplay)