UFO:Alien Invasion Code

* reverted 'removed icons and buttons that are part of the icon pack'

* This reverts commit 0069eab13c03896625189628c762040d1f983b16.
* this wasn't for the master, but for the 2.4 branch

* removed icons and buttons that are part of the icon pack

* i also had to remove the LICENSES entries to make the commit hook happy - grab the license information either from this commit or from master

* Reduced filesizes of the buttons pictures folder by converting the files to 8bit/256 indexed colors and afterwards compressing/optimizing the files with pngout

* split button_background.png and update custombutton@...