New lfun to copy index insets from ToC

New lfun to copy index insets from ToC

The new function inset-insert-copy can only be invoked from the ToC.
It is currently only implemented for Index inset.

It is a special lfun because Inset::dispatch is called directly from
the ToC widget with cursor pointing to the inset, whereas the patch
happens in the workarea at caret position. This function cannot be
called directly.

Add an entry for this function in the toc context menu.

Fixes bug #4582.