Add the desktop file for the snap

Now that we have added plugs to allow accessing the display, the snap needs
a desktop file. And bonus point, it will appear on the menu when it's
installed, and once you make a stable relase, it will appear in the gnome
software center app! So, one-click install for parity :)

Closes: #7056

snap/gui/parity.desktop

@@ -0,0 +1,8 @@
+[Desktop Entry]
+Type=Application
+Encoding=UTF-8
+Name=parity
+Comment=Fast, light, robust Ethereum implementation
+Exec=parity
+Icon=${SNAP}/meta/gui/icon.png
+Terminal=true