desktop file: drop exec and fix it according to the spec (#582)

Fixes #580, fixes #581

Signed-off-by: Jaroslav Škarvada <jskarvad@redhat.com>

Signed-off-by: Jaroslav Škarvada <jskarvad@redhat.com>

NanoVNASaver.desktop

@@ -1,19 +1,16 @@
 [Desktop Entry]
-Categories=Electronics
+Categories=Electronics;Education;
 Comment[de_DE]=Programm das Daten vom NanoVNA liest, anzeigt und speichert
 Comment=Tool for reading, displaying and saving data from the NanoVNA
-Encoding=UTF-8
 Exec=NanoVNASaver
 GenericName[de_DE]=
 GenericName=
-Icon=NanoVNASaver_48x48.png
+Icon=NanoVNASaver_48x48
 MimeType=
 Name[de_DE]=NanoVNASaver
 Name=NanoVNASaver
-Path=
 StartupNotify=true
 Terminal=false
-TerminalOptions=
 Type=Application
 X-DBUS-ServiceName=
 X-DBUS-StartupType=