GUI picture overhaul.

* Glass buttons. These new buttons are in the SVG format in source/images if you wish to make derived skins.
* Icons from the Tango Icon Library and the RRZE Icon Set. These new icons are in the SVG format in source/images if you wish to make derived skins.
* Main menu: More readable text on unselected options.
* Main menu: Make "New game", "Return to game" and "Reset game" localised in English, French and Simplified Chinese. These three options also all use the same button image now; snewo, snnewo, sreseto, snreseto, sreteno and snreteno are gone and replaced by smaini and snmaini.
* copyright: Update.
* Align main menu icons and text better.
ICON_BUTTON is index 36, not 46.
Change the "back" icon.
Remove a quasi-green pixel that was showing up in main menu selected items.