Given that the per-view callback is in SfxViewShell, and editeng doesn't
depend on sfx2, add an interface class in editeng to invert the
dependency.
With this, gtktiledviewer no longer crashes when starting editeng text
edit with per-view callbacks.
Change-Id: I783cdc646b890a6b597000f1d88428c8042417cf
Reviewed-on: https://gerrit.libreoffice.org/26169
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>
