Improvement: Change window type detection

- Let window type detection start with the client window if there's one,
  in hope to enhance performance.

- Change get_wintype_prop() to fetch the property only once.

- Default to WINTYPE_UNKNOWN instead of WINTYPE_NORMAL if
  _NET_WM_WINDOW_TYPE is missing.

- Fix a mistake in calc_opacity().

- Add some items to .gitignore.

- Fix a typo in usage().

.gitignore

@@ -15,6 +15,13 @@ stamp-h1
 compton
 *.o
 *~
+core
 
 # Vim files
 .*.sw[a-z]
+
+# oprofile files
+oprofile_data
+
+# clang files
+compton.plist