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

CMakeLists.txt

@@ -178,7 +178,7 @@ if(UNIX AND NOT APPLE)
 	install(FILES "doc/minetest.6" "doc/minetestserver.6" DESTINATION "${MANDIR}/man6")
 	install(FILES "misc/minetest.desktop" DESTINATION "${XDG_APPS_DIR}")
 	install(FILES "misc/minetest.appdata.xml" DESTINATION "${APPDATADIR}")
-	install(FILES "misc/minetest-icon.svg" DESTINATION "${ICONDIR}/hicolor/scalable/apps")
+	install(FILES "misc/minetest.svg" DESTINATION "${ICONDIR}/hicolor/scalable/apps")
 endif()
 
 if(APPLE)