make submap deal bindings flexible

[vincentaxhe]

Description

bind = CTRL, Tab, changegroupactive, f
bind = SHIFT, Tab, changegroupactive, b
bind = $mainMod ALT, Tab, submap, firefoxtab
submap = firefoxtab
	bind = ,escape,submap,reset
	bind = ,return,submap,reset
submap = reset

I try to use ctrl+tab and shift+tab both in group and firefox, enter an empty submap in order to disable these two bindings which will disable all keybings, there must some flexible way to make some bindings cancellable. other scenarios like use alt + [a-z] globally but cancel them under gui with alt invoked menus