[tc_editline] Add callbacks for tab completion and hints

[darkuranium]

Currently, only a single callback exists. Instead, multiple events should be recognized and passed to the user.

I have already decided to go with option nr. 3, but for the record, there were three options:

1. N callbacks registered separately

   void tcedit_register_tabcomplete(int (*cb)(<...>), void* udata);
   void tcedit_register_echo(<...>, void* udata);
   

2. N callbacks registered at once (via a struct)

   typedef struct TC_EditHandler {
       int (*tabcomplete)(<...>);
       <...>
       void* udata;
   } TC_EditHandler;
   void tcedit_register_handler(TC_EditHandler* handler);
   

3. A single callback which is passed an event type.

   typedef struct TC_EditEvent {
       int type;
       TC_String* input;
       <...>
   } TC_EditEvent;
   typedef int TC_EditCallback(TC_EditEvent* event, void* udata);
   void tcedit_register_callback(TC_EditCallback* callback, void* udata);