Implement the configuration option floating_modifier and document it

include/config.h

@@ -3,7 +3,7 @@
  *
  * i3 - an improved dynamic tiling window manager
  *
- * (c) 2009 Michael Stapelberg and contributors
+ * © 2009 Michael Stapelberg and contributors
  *
  * See file LICENSE for license information.
  *
@@ -37,6 +37,10 @@ struct Config {
         const char *terminal;
         const char *font;
 
+        /** 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;
+
         /* Color codes are stored here */
         struct config_client {
                 struct Colortriple focused;