add OverlapGroup in widget.lua

frontend/ui/widget.lua

@@ -478,6 +478,56 @@ function VerticalGroup:free()
 	WidgetContainer.free(self)
 end
 
+
+--[[
+A Layout widget that puts objects above each other
+]]
+OverlapGroup = WidgetContainer:new{
+	_size = nil,
+}
+
+function OverlapGroup:getSize()
+	if not self._size then
+		self._size = {w = 0, h = 0}
+		self._offsets = { x = math.huge, y = math.huge }
+		for i, widget in ipairs(self) do
+			local w_size = widget:getSize()
+			if self._size.h < w_size.h then
+				self._size.h = w_size.h
+			end
+			if self._size.w < w_size.w then
+				self._size.w = w_size.w
+			end
+		end
+	end
+
+	if self.dimen.w then
+		self._size.w = self.dimen.w
+	end
+	if self.dimen.h then
+		self._size.h = self.dimen.h
+	end
+
+	return self._size
+end
+
+function OverlapGroup:paintTo(bb, x, y)
+	local size = self:getSize()
+	
+	for i, wget in ipairs(self) do
+		local wget_size = wget:getSize()
+		if wget.align == "right" then
+			wget:paintTo(bb, x+size.w-wget_size.w, y)
+		elseif wget.align == "center" then
+			wget:paintTo(bb, x+(size.w-wget_size.w)/2, y)
+		else
+			-- default to left
+			wget:paintTo(bb, x, y)
+		end
+	end
+end
+
+
 --[[
 Dummy Widget that reserves vertical space
 ]]