Feature #65: Auxiliary window detection

- Add auxiliary window detection (--detect-transient &
  --detect-client-leader). Thanks to SmilingHorse for inspiring me. The
  implementation is not too speed-efficient, and bugs are to be
  expected.

- Known issue: auxiliary window detection may not work too well with
  windows that are never mapped, for example as client leader.

compton.sample.conf

@@ -43,6 +43,8 @@ paint-on-overlay = false;
 sw-opti = false;
 unredir-if-possible = false;
 focus-exclude = [ ];
+detect-transient = true;
+detect-client-leader = true;
 
 # Window type settings
 wintypes: