This is similar to commit f1e77547
(framework: allow storeSelf() on the main thread, 2019-01-30), just this
handles "save as" instead of "save".

The result is that combining this commit with the previous OnMainThread
ones allows all of document load/save/save-as/command-dispatch on the
main thread even when the action is invoked via remote UNO, which would
run on a non-main thread by default.

Change-Id: I7d50cceb66ecc6619fe25734107a2524ca872c2a
Reviewed-on: https://gerrit.libreoffice.org/67412Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins