[UX] Gesture manager: add diagonal two-finger swipes (#4820)
[UX] Gesture manager: add diagonal two-finger swipes (#4820)

Follow-up to #4815. Pointed out as an easy target [here](https://github.com/koreader/koreader/pull/4815#issuecomment-473990141) by @poire-z.
mihailim/koreader,Frenzie/koreader,Markismus/koreader,houqp/koreader,mwoz123/koreader,Frenzie/koreader,poire-z/koreader,NiLuJe/koreader,pazos/koreader,koreader/koreader,Hzj-jie/koreader,poire-z/koreader,NiLuJe/koreader,koreader/koreader