ui.backend.gtk: better gesture for shift tab
ui.backend.gtk: better gesture for shift tab

gdk emits ISO_Left_Tab for shift + tab. Not sure if factor should silently
transform that in tab..
With this patch, shift tabs emits the following gesture:
T{ key-down { mods { S+ } } { sym "TAB" } }

mcandre/factor,mcandre/factor,mcandre/factor,mcandre/factor,mcandre/factor,mcandre/factor,mcandre/factor