nano_wallet: add desktop file

[milahu]

nano_wallet is a gui app, so it should have a desktop file

nano-node version d8d564bfdb79195caaf9e420a3c14f952fd6783a does not install a desktop file for nano_wallet

expected: similar to /share/applications/monero-wallet-gui.desktop

[Desktop Entry]
Categories=Network;Utility
Exec=monero-wallet-gui
GenericName=Wallet
Icon=monero
Name=Monero
Type=Application
Version=1.4

[SparK-Cruz]

I still don't know why the node contains a wallet and a gui for it. Maybe we should open another repo for it and remove it from this one.