ReaderCropping: prettier bottom buttons (#6900)
ReaderCropping: prettier bottom buttons (#6900)

Also remove the grey gap between the page and the buttons
that would appear only when the footer is visible.
NiLuJe/koreader,Markismus/koreader,Hzj-jie/koreader,pazos/koreader,poire-z/koreader,poire-z/koreader,NiLuJe/koreader,Frenzie/koreader,mwoz123/koreader,koreader/koreader,Frenzie/koreader,koreader/koreader