Epiphany Freeze Break request

Hi,

I would like to ask for a freeze break request to improve navigation 
gestures in Epiphany by using gesture from WebKit instead of a custom 
implementation.

The MR[1] removes custom gesture implementation, toggles WebKit 
settings property to enable the gesture, and adds it to shortcuts 
dialog. Finally, it reduces shortcuts dialog height because of the 
added gesture, going from [2] to [3].

The MR introduces no new strings.

The feature has not been introduced before because the corresponding 
WebKit release (2.23.90) is not yet out as of the time of writing.

[1] https://gitlab.gnome.org/GNOME/epiphany/merge_requests/183
[2] https://imgur.com/a/gYeITyQ
[3] https://imgur.com/a/QvWEv46