Implemented GameConnection APIs.

1. Bridge now passes along ISteamUser's two GameConnection API calls.
2. Note that Wineline documentation suggests 16-bit parameters aren't
valid in win32, persumably being upsized to 32-bit integers.  In practice,
this seems to work correctly.
Updated bridge spec for two new API calls.
Added better callback tracking and can shutdown.

1. Implemented SteamAPI_Shutdown.
2. Context now tracks which wrappers correspond to which callbacks.
3. Added basic support for removing a callback.
Implemented callback removal, + cleanup.

1. Cleanup/polishing of various minor parts of callback code.  Better
TRACEs, variable names, formatting, etc.
2. Implemented SteamAPI_UnregisterCallback in the bridge!