Using timestamped events for setting xinsert callbacks

The a2_Set*Callback() calls now send timestamped events, so
callbacks are installed when intended. This also avoids the
problem with installing callbacks before the xinsert units
have actually been instantiated, as the events will now be
processed after the VM has executed the INITV instruction.

Added some more realtime error handling to the message/event
handling.

Added A2_NOOBJECT error code.

Removed the temporary SDL_Delay() hack in a2test.c.

Closes #77.