separate abstract interface EventListener from Widget

The rationale is that some non-widget modules like ReaderKoptListener should be able
to handle events.

frontend/ui/widget.lua

@@ -7,6 +7,31 @@ require "ui/inputevent"
 require "ui/gesturedetector"
 require "ui/font"
 
+--[[
+The EventListener is an interface that handles events
+
+EventListeners have a rudimentary event handler/dispatcher that
+will call a method "onEventName" for an event with name
+"EventName"
+
+]]
+EventListener = {}
+
+function EventListener:new(o)
+	local o = o or {}
+	setmetatable(o, self)
+	self.__index = self
+	if o.init then o:init() end
+	return o
+end
+
+function EventListener:handleEvent(event)
+	if self[event.handler] then
+		return self[event.handler](self, unpack(event.args))
+	end
+end
+
+
 --[[
 This is a generic Widget interface
 
@@ -18,7 +43,7 @@ if the table that was given to us as parameter has an "init"
 method, it will be called. use this to set _instance_ variables
 rather than class variables.
 ]]
-Widget = {}
+Widget = EventListener:new()
 
 function Widget:new(o)
 	local o = o or {}
@@ -40,19 +65,6 @@ end
 function Widget:paintTo(bb, x, y)
 end
 
---[[
-Widgets have a rudimentary event handler/dispatcher that
-will call a method "onEventName" for an event with name
-"EventName"
-
-These methods
-]]
-function Widget:handleEvent(event)
-	if self[event.handler] then
-		return self[event.handler](self, unpack(event.args))
-	end
-end
-
 --[[
 WidgetContainer is a container for another Widget
 ]]