Separate border width for floating windows

Floating windows already had their own border style, but the width was
the same for all windows.

The configuration directives 'new_window' and 'new_float' can now be
used simultaneously to have different border widths for floating and
tiled windows.

fixes #1244

include/config.h

@@ -98,6 +98,7 @@ struct Config {
     int container_stack_limit;
     int container_stack_limit_value;
     int default_border_width;
+    int default_floating_border_width;
 
     /** Default orientation for new containers */
     int default_orientation;