Refine game::debugmon:

Rename to debug_mode, that's what it's called when you toggle it from the debug menu.

Moved it to the debug.{cpp,h} files as it is a debug, not a game feature.

Added a message type m_debug, which is *only* shown when the debug mode is activated.
Replaced "if(debug_mode) { add_msg(); }" with simpler "add_msg(m_debug, ...)"
The message class will handle the display of those messages, or it suppresses them if the debug mode is not activated.

Renamed ACTION_TOGGLE_DEBUGMON to ACTION_TOGGLE_DEBUG_MODE to match the actual action name.
Also renamed the visible name of that action in the key binding menu.