Rename minetest-icon.svg to minetest.svg - Fixes #1564

-Complies with the Linux standard freedesktop icon naming specs: http://standards.freedesktop.org/icon-naming-spec/icon-naming-spec-latest.html#guidelines
-Ensures compatibility with icon themes
-Edit minetest.desktop, CMakeLists.txt and README.md to reflect this change

misc/minetest.desktop

@@ -9,7 +9,7 @@ Comment[ja]=マルチプレイに対応した、無限の世界のブロック
 Comment[ru]=Игра-песочница с безграничным миром, состоящим из блоков
 Comment[tr]=Tek-Çok oyuncuyla küplerden sonsuz dünyalar inşa et
 Exec=minetest
-Icon=minetest-icon
+Icon=minetest
 Terminal=false
 Type=Application
 Categories=Game;

misc/minetest.svg

@@ -14,7 +14,7 @@
    id="svg2856"
    version="1.1"
    inkscape:version="0.47 r22583"
-   sodipodi:docname="minetest-icon.svg"
+   sodipodi:docname="minetest.svg"
    inkscape:export-filename="/home/erlehmann/pics/icons/minetest/minetest-icon-24x24.png"
    inkscape:export-xdpi="45"
    inkscape:export-ydpi="45">