Make it pass desktop-file-validate, closes #1784

```
cura.desktop: error: (will be fatal in the future): value "cura-icon.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
```
__Solution:__ Remove `.png`. __PLEASE NOTE__ that for this to work, cura-icon.png must be located in one of the directories in which desktops search for icons, such as `/usr/share/icons/hicolor/128x128/apps/`. Please make sure it ends up there (also in the AppImage, too.)

```
cura.desktop: error: value "application/sla;application/vnd.ms-3mfdocument;application/prs.wavefront-obj;image/bmp;image/gif;image/jpeg;image/png" for string list key "MimeType" in group "Desktop Entry" does not have a semicolon (';') as trailing character
```
__Solution:__ Add `;`

Reference:
https://travis-ci.org/AppImage/AppImageHub/builds/266072586#L538-L539

cura.desktop.in

@@ -7,9 +7,9 @@ Comment=Cura converts 3D models into paths for a 3D printer. It prepares your pr
 Comment[de]=Cura wandelt 3D-Modelle in Pfade für einen 3D-Drucker um. Es bereitet Ihren Druck für maximale Genauigkeit, minimale Druckzeit und guter Zuverlässigkeit mit vielen zusätzlichen Funktionen vor, damit Ihr Druck großartig wird.
 Exec=@CMAKE_INSTALL_FULL_BINDIR@/cura %F
 TryExec=@CMAKE_INSTALL_FULL_BINDIR@/cura
-Icon=@CMAKE_INSTALL_FULL_DATADIR@/cura/resources/images/cura-icon.png
+Icon=cura-icon
 Terminal=false
 Type=Application
-MimeType=application/sla;application/vnd.ms-3mfdocument;application/prs.wavefront-obj;image/bmp;image/gif;image/jpeg;image/png
+MimeType=application/sla;application/vnd.ms-3mfdocument;application/prs.wavefront-obj;image/bmp;image/gif;image/jpeg;image/png;
 Categories=Graphics;
 Keywords=3D;Printing;