Legacy plug-ins like Checkerboard (legacy) under Render Patterns lost
their Help button, although using F1 was still working.

Due to changes in how procedures are called, `gimp_dialog_constructed`
was called before the help-id was set.
The simplest way to fix this, is to call
 `G_OBJECT_CLASS (parent_class)->constructed (object);`
at a later time in `gimp_procedure_dialog_constructed` after we have
set the help-id there.