add content range method in BottomContainer

frontend/ui/widget/container/bottomcontainer.lua

@@ -1,4 +1,5 @@
 local WidgetContainer = require("ui/widget/container/widgetcontainer")
+local Geom = require("ui/geometry")
 
 --[[
 BottomContainer contains its content (1 widget) at the bottom of its own
@@ -17,4 +18,14 @@ function BottomContainer:paintTo(bb, x, y)
 		y + (self.dimen.h - contentSize.h))
 end
 
+function BottomContainer:contentRange()
+	local contentSize = self[1]:getSize()
+	return Geom:new{
+		x = (self.dimen.x or 0) + math.floor((self.dimen.w - contentSize.w)/2),
+		y = (self.dimen.y or 0) + self.dimen.h - contentSize.h,
+		w = contentSize.w,
+		h = contentSize.h
+	}
+end
+
 return BottomContainer