Fixed .desktop file conformity

Now that we use the included .desktop file instead of our own it is time to fix RPMlint errors upstream:

```
s25rttr.i586: W: invalid-desktopfile /usr/share/applications/s25rttr.desktop file contains at least one line ending with a carriage return, while lines should only be separated by a line feed character. First such line is: "[Desktop Entry]"
s25rttr.i586: W: invalid-desktopfile /usr/share/applications/s25rttr.desktop (will be fatal in the future): value "s25rttr.png" for key "Icon" in group "Desktop Entry" is an icon name with an extension, but there should be no extension as described in the Icon Theme Specification if the value is not an absolute path
.desktop file is not valid, check with desktop-file-validate
```