[UX] Gesture manager: delete custom gestures and update menu (#4652)
[UX] Gesture manager: delete custom gestures and update menu (#4652)

Include a safety check to prevent rerecording an existing gesture.

Also extend the default set of gestures to include all of them, and add east south west north for refresh. (I've always thought small diagonal swipe to be rather awkward to trigger.)

See https://github.com/koreader/koreader/pull/4644#issuecomment-466743715 and https://github.com/koreader/koreader/issues/4651 regarding the menu update.
NiLuJe/koreader,poire-z/koreader,Markismus/koreader,mwoz123/koreader,NiLuJe/koreader,mihailim/koreader,poire-z/koreader,koreader/koreader,Hzj-jie/koreader,pazos/koreader,koreader/koreader,Frenzie/koreader,houqp/koreader,Frenzie/koreader