Set GNOME single window property

[HebaruSan]

Problem

In GNOME Shell currently, if you launch CKAN via the system menu, its icon has a "New Window" option:

Clicking it launches CKAN again, which invariably brings up the locked instance error:

It is a little embarrassing to have a highly visible option that doesn't work well by default.

Noticed while investigating #3423.

Cause

The "New Window" option is added by GNOME Shell automatically (if the .desktop file's Actions= list doesn't include new-window), and probably works fine for 90% of apps where running the Exec again successfully opens a new window that works properly.

Changes

Now our .desktop files set X-GNOME-SingleWindow=true to suppress the "New Window" option.

To find out this option existed, I had to read the GNOME source code:

• https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/main/src/shell-app.c#L662-665

I looked for a similar option for KDE or other desktops but could not find one, but I'm happy to add it here if it exists.

In addition, StartupNotify is removed because we do not "KNOW" that our application will send this message (because who knows what Mono's WinForms will do or not do in any situation):

• https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html#recognized-keys

  If true, it is KNOWN that the application will send a "remove" message when started with the DESKTOP_STARTUP_ID environment variable set. If false, it is KNOWN that the application does not work with startup notification at all (does not shown any window, breaks even when using StartupWMClass, etc.). If absent, a reasonable handling is up to implementations (assuming false, using StartupWMClass, etc.). (See the Startup Notification Protocol Specification for more details).

[DasSkelett]

Nope, can't find anything for KDE either. And X-GNOME-SingleWindow=true or X-KDE-SingleWindow=truedon't have an effect.

Also not testing on Gnome, just gonna believe your screenshots there 😆