add flipping mode in pdf/djvu reader

By tapping on the upper left corner of the reader one can toggling flipping mode on/off.
In flipping mode swiping right/down and left/up  will paging backward and forward
accordingly with pages proportional to swiping distance.

frontend/ui/reader/readerflip.lua

@@ -0,0 +1,16 @@
+
+ReaderFlipping = LeftContainer:new{}
+
+function ReaderFlipping:init()
+	local widget = ImageWidget:new{
+		file = "resources/icons/appbar.book.open.png",
+	}
+	local icon_size = widget:getSize()
+	self.dimen = Geom:new{w = Screen:getWidth(), h = icon_size.h}
+	self[1] = widget
+end
+
+function ReaderFlipping:onSetDogearVisibility(visible)
+	self.view.dogear_visible = visible
+	return true
+end