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/backend/gl/gl_common.c

@@ -50,7 +50,7 @@ struct gl_blur_context {
 static GLint glGetUniformLocationChecked(GLuint p, const char *name) {
 	auto ret = glGetUniformLocation(p, name);
 	if (ret < 0) {
-		log_error("Failed to get location of uniform '%s'. compton might not "
+		log_error("Failed to get location of uniform '%s'. the compositor might not "
 		          "work correctly.",
 		          name);
 	}