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/utils.h

@@ -78,7 +78,7 @@ safe_isnan(double a) {
 
 // Some macros for checked cast
 // Note these macros are not complete, as in, they won't work for every integer types. But
-// they are good enough for compton.
+// they are good enough for our use cases.
 
 #define to_int_checked(val)                                                              \
 	({                                                                               \