Makes the Dock icon only appear when a script plugin has a dialog.
Unfortunately, on MacOS where "transient for" doesn't work,
we need a Dock icon for the separate process so user can "Show window."
See #12150
Done at runtime.
A build-time alternative is: package script-fu binary in a bundle, and set properties.
Fixes both the new-style dialog (GimpProcedureDialog) and the old-style dialog (deprecated.)
e56f2b1c54