Desktop: Add "X-GNOME-SingleWindow=true" to the desktop file (#5948)

Since this commit (https://github.com/KDE/plasma-workspace/commit/ebd2acd98ecdb9d115b01bd01f532390376152f7),
Plasma Desktop will check "X-GNOME-SingleWindow" property to determine
whether to show "Open New Window" action in the context menu of a task.

Joplin cannot launch a new instance or open a new window, so add the
property and set it to true to hide the action.