Warn when close Marv it will be minimized

[syrm]

When you close Marv it is in fact minimized, but users may don't know that.
Warn the user the first time he "closes" Marv it will be minimized.

[skarab42]

I will add this in v1.x

[j-wiedemann]

There is now a quit button that kill marv server in the top left menu 👍