Add lowmemory callback

Author: rm-code

Screen.lua

@@ -53,6 +53,8 @@ function Screen.new()
 
     function self:keyreleased(key) end
 
+    function self:lowmemory() end
+
     function self:textinput(input) end
 
     function self:mousereleased(x, y, button) end

ScreenManager.lua

@@ -198,6 +198,13 @@ function ScreenManager.keyreleased(key)
     ScreenManager.peek():keyreleased(key);
 end
 
+---
+-- Callback function triggered when the system is running out of memory on mobile devices.
+--
+function ScreenManager.lowmemory()
+    ScreenManager.peek():lowmemory();
+end
+
 ---
 -- Reroute the textinput callback to the currently active screen.
 -- @param input