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/opengl.h

@@ -40,7 +40,7 @@ typedef struct {
 	GLint unifm_factor_center;
 } glx_blur_pass_t;
 
-/// Structure containing GLX-dependent data for a compton session.
+/// Structure containing GLX-dependent data for a session.
 typedef struct glx_session {
 	// === OpenGL related ===
 	/// GLX context.