Previously, the arc was entirely created in the GuiCommand
definition. The code to create only the arc is placed inside
a module so that the function can be used in scripting
from the terminal, without a graphical interface.
Inside the class `_DraftLink` the `onDocumentRestored` function
tests for the existence of the old attribute `useLink`. If it is
present, it uses it to define the new variable name `use_link`.
This is done use Python naming conventions.
The old `useLink` is deleted so it is not longer saved
together with the object when the document is saved.
It replaces the older `Draft_Array` command, and provides
a task panel to select the properties, similar to the
`Draft_PolarArray` and `Draft_CircularArray` commands.
It can also create `App::Links` directly from this task
panel, so it also replaces the `Draft_LinkArray`
introduced by the LinkMerge.
