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/event.c

@@ -11,7 +11,7 @@
 #include "atom.h"
 #include "common.h"
 #include "compiler.h"
-#include "compton.h"
+#include "picom.h"
 #include "config.h"
 #include "event.h"
 #include "log.h"
@@ -633,7 +633,7 @@ ev_selection_clear(session_t *ps, xcb_selection_clear_event_t attr_unused *ev) {
 	// If we lose that one, we should exit.
 	log_fatal("Another composite manager started and took the _NET_WM_CM_Sn "
 	          "selection.");
-	quit_compton(ps);
+	quit(ps);
 }
 
 void ev_handle(session_t *ps, xcb_generic_event_t *ev) {