Add new_float config option.

This option sets the default border style for containers automatically put into floating mode.

Fixes #264

include/config.h

@@ -126,6 +126,9 @@ struct Config {
         /** The default border style for new windows. */
         border_style_t default_border;
 
+        /** The default border style for new floating windows. */
+        border_style_t default_floating_border;
+
         /** The modifier which needs to be pressed in combination with your mouse
          * buttons to do things with floating windows (move, resize) */
         uint32_t floating_modifier;