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" } }

bpollack/factor,bjourne/factor,mrjbq7/factor,mrjbq7/factor,sarvex/factor-lang,AlexIljin/factor,bpollack/factor,dch/factor,AlexIljin/factor,bjourne/factor,dch/factor,factor/factor,sarvex/factor-lang,slavapestov/factor,nicolas-p/factor,dch/factor,slavapestov/factor,sarvex/factor-lang,nicolas-p/factor,mrjbq7/factor,nicolas-p/factor,AlexIljin/factor,bpollack/factor,bpollack/factor,factor/factor,nicolas-p/factor,dch/factor,tgunr/factor,bjourne/factor,mrjbq7/factor,bjourne/factor,factor/factor,dch/factor,tgunr/factor,sarvex/factor-lang,mrjbq7/factor,bjourne/factor,AlexIljin/factor,factor/factor,nicolas-p/factor,tgunr/factor,sarvex/factor-lang,slavapestov/factor,factor/factor,AlexIljin/factor,tgunr/factor,slavapestov/factor,slavapestov/factor,bjourne/factor,bjourne/factor,nicolas-p/factor,slavapestov/factor,dch/factor,bpollack/factor,mrjbq7/factor,AlexIljin/factor,sarvex/factor-lang,nicolas-p/factor,sarvex/factor-lang,bpollack/factor,slavapestov/factor,bpollack/factor,tgunr/factor,tgunr/factor,AlexIljin/factor,factor/factor