desktop-entry-spec: Add "SingleMainWindow" key
This is an upstreaming of the GNOME-specific X-GNOME-SingleWindow key, which functions as the same thing, signaling that an app only supports having a single one of its main window open.
We recently added support for apps with that key in KDE [1] and thought it would be useful to upstream (with a slightly more accurate name, since technically single-instance apps can open multiple windows) so that everyone could benefit from the same thing.
[1] https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/1309
Edited by Nate Graham