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/win.h

@@ -73,7 +73,7 @@ struct window_stack_entry {
  *        considered part of the window.
  */
 
-/// Structure representing a top-level window compton manages.
+/// Structure representing a top-level managed window.
 typedef struct win win;
 struct win {
 	UT_hash_handle hh;