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

parser-specs/config.spec

@@ -453,7 +453,7 @@ state BAR_ID:
       -> call cfg_bar_id($bar_id); BAR
 
 state BAR_MODIFIER:
-  modifier = 'Mod1', 'Mod2', 'Mod3', 'Mod4', 'Mod5', 'Control', 'Ctrl', 'Shift'
+  modifier = 'Mod1', 'Mod2', 'Mod3', 'Mod4', 'Mod5', 'Control', 'Ctrl', 'Shift', 'none', 'off'
       -> call cfg_bar_modifier($modifier); BAR
 
 state BAR_WHEEL_UP_CMD: