Add support for new blur method dual_kawase [WIP]

**Work-in-Progress**

Add `dual_kawase` to configuration and argument parsing. Allow `kawase`
for backward compatibility. Add `--blur-strength` parameter for
blur-method `dual_kawase`.

Update documentation to reflect the new blur-method and parameters.

picom.sample.conf

@@ -143,6 +143,8 @@ focus-exclude = [ "class_g = 'Cairo-clock'" ];
 # blur-size = 12
 #
 # blur-deviation = false
+#
+# blur-strength = 5
 
 # Blur background of semi-transparent / ARGB windows. 
 # Bad in performance, with driver-dependent behavior.