Add StartupWMClass to spotify_spotify.desktop file in the snap package

[113587947]

This property allows the window manager to keep only one icon for all the windows opened by the application. Since snap actually runs itself, which in turn runs spotify, then users end up with two icons in the Dock, which is quite annoying.

It is a very simple thing to do, just add

StartupWMClass=spotify

to the file installed at

/var/lib/snapd/desktop/applications/spotify_spotify.desktop

I did it manually and it works, however I'm sure that after the next update this will be lost and I'll have to do it again.

Thank you,

Leonardo