Allow "modifier none" in i3bar to disable the modifier.

This allows "modifier none" (and "modifier off") for the bar config
in order to disable the modifier key altogether. This is useful
for users who use a different approach to hiding / showing the bar,
e.g., a custom keybind that involved multiple keys or scripts.

fixes #2208

docs/userguide

@@ -1238,7 +1238,7 @@ the windows key). The default value for the hidden_state is hide.
 -------------------------
 mode dock|hide|invisible
 hidden_state hide|show
-modifier <Modifier>
+modifier <Modifier>|none
 ------------------------
 
 *Example*:
@@ -1250,7 +1250,8 @@ bar {
 }
 ----------------
 
-Available modifiers are Mod1-Mod5, Shift, Control (see +xmodmap(1)+).
+Available modifiers are Mod1-Mod5, Shift, Control (see +xmodmap(1)+). You can
+also use "none" if you don't want any modifier to trigger this behavior.
 
 === Mouse button commands