Backport of zotero/zotero#4163

The SingleMainWindow key might be upstream, but it is still not used in most systems.

X-GNOME-SingleWindow was upstreamed to be an XDG spec with the name
"SingleMainWindow" in
https://gitlab.freedesktop.org/xdg/xdg-specs/-/merge_requests/53

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.

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

Closes #81
Closes #86

https://forums.zotero.org/discussion/85872/patch-zotero-desktop-file-does-not-work-when-filename-is-provided-as-a-uri

Adds a valid category https://standards.freedesktop.org/menu-spec/latest/apa.html to improve Linux visibility, fixes #63

Since 'zotero' is now a shell script that might open in a text editor by
default (as it does on Ubuntu), add a launcher (.desktop) file that can
be clicked on by tarball users. .desktop files stupidly can't use
relative paths, so use a hack to run the executable and add a script
that will update the .desktop file if it's writable with the current
path to the icon. Technically the .desktop file could call the script to
update itself, but it wouldn't take effect until the second launch
anyway and can possibly cause the launcher icon to behave strangely, so
let people run it manually if they care. They should really be using a
package anyway.