Fixed .desktop file conformity

from @opensuse games:

```
RPMLINT report:
===============
hedgewars.i586: W: no-manual-page-for-binary hwengine
Each executable in standard binary directories should have a man page.

hedgewars.i586: W: invalid-desktopfile /usr/share/applications/hwengine.desktop (will be fatal in the future): value "hedgewars.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
hedgewars.i586: W: invalid-desktopfile /usr/share/applications/hwengine.desktop value "application/x-hedgewars-demo;application/x-hedgewars-save" for string list key "MimeType" in group "Desktop Entry" does not have a semicolon (';') as trailing character
hedgewars.i586: W: invalid-desktopfile /usr/share/applications/hedgewars.desktop (will be fatal in the future): value "hedgewars.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
hedgewars.i586: W: invalid-desktopfile /usr/share/applications/hedgewars.desktop value "x-scheme-handler/hwplay" for string list key "MimeType" in group "Desktop Entry" does not have a semicolon (';') as trailing character
 .desktop file is not valid, check with desktop-file-validate
```