Set icon for app window

[mpsijm]

Note that the desktop icon is working fine, but I'm missing the icon in the window button in my XFCE panel:

The icon would probably need to be added to the BrowserWindow here: https://github.com/gristlabs/grist-desktop/blob/main/ext/app/electron/WindowManager.ts#L56

I just don't know exactly how to import the icon(s) from https://github.com/gristlabs/grist-core/tree/main/static/icons. It's probably as simple as setting a path using setIcon or passing it as option to the constructor, but I don't know what the path should be, as the icons live in a folder outside ext/app.

Note that this change will create a (simple) conflict with #42, which moves the creation of the BrowserWindow to a different function. EDIT: #42 is merged now, updated the file path 😄