Add SingleMainWindow support for Linux items

[larsoner]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

To set the icon of Qt applications properly on latest Ubuntu I need to create the following .desktop file:

[Desktop Entry]
Type=Application
Version=1.5
Name=MNE-Python
StartupWMClass=MNE-Python
Icon=/home/larsoner/python/mne-python/mne/icons/mne_default_icon.png
SingleMainWindow=true
NoDisplay=true

This isn't meant to be a launcher, just create an association between the given Qt window's StartupWMClass and an Icon. There doesn't seem to be any runtime Qt method to get the icon to change, so without this .desktop file we're stuck with the default gear icon (ignore the Signal icon above it):

All the options are there except the SingleMainWindow one, which is helpful because it takes this default behavior of having a "New Window" prompt:

And allows it to be removed, which is what I want:

Why is this needed?

Anyone who wants to remove the "New Window" option on latest Ubuntu will benefit from this feature.

What should happen?

☝

Additional Context

N/A