rename: replace "compton" in the codebase

leftovers:

1) config file path. Has to implement compatibility functionalities before
  we can change it.

2) links in man pages. Has to migrate the repo first.

3) _COMPTON_SHADOW, it has become a defacto standard, so we have to keep
   supporting it.

4) dbus names, undecided whether we should/could change it.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/c2.c

@@ -1028,7 +1028,7 @@ static bool c2_l_postprocess(session_t *ps, c2_l_t *pleaf) {
 		}
 	}
 
-	// Enable specific tracking options in compton if needed by the condition
+	// Enable specific tracking options, if needed by the condition
 	// TODO: Add track_leader
 	switch (pleaf->predef) {
 	// case C2_L_PROUNDED: ps->o.detect_rounded_corners = true; break;