Added MouseInputSystem and updated GUI System

Implemented a MouseInput System, that allows access to the Mouse input.
This is used by the Clickable Component, once you click on an Entity
that has a Clickable Component the onclick "command" is executed.
Current commands are:
openpage:<page> Open the page <page>
closepage:<page> Close the page <page>
exitprogram Closes the program
A page is defined in the Drawable component, only those elements that
are part of a page that is currently displayed are drawn or can be
clicked. The "default" page is the only one shown at the start.
That is being shown with the "exit" button.
Implemented a simple blending and back to front rendering.

test.sc has been split up into default.sc and game.sc to prepare for a
loading/new game implementation, by implementing dynamic loading and
unloading of sc-files.