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/config.h

@@ -28,7 +28,7 @@
 
 typedef struct session session_t;
 
-/// @brief Possible backends of compton.
+/// @brief Possible backends
 enum backend {
 	BKEND_XRENDER,
 	BKEND_GLX,
@@ -226,9 +226,9 @@ typedef struct options {
 	bool detect_client_leader;
 
 	// === Calculated ===
-	/// Whether compton needs to track window name and class.
+	/// Whether we need to track window name and class.
 	bool track_wdata;
-	/// Whether compton needs to track window leaders.
+	/// Whether we need to track window leaders.
 	bool track_leader;
 
 	// Don't use EWMH to detect fullscreen applications