Like the other values with a special meaning
when passed to SfxObjectShell::GetTitle use
predefined constants, also use the already
defined SFX_TITLE_TITLE constant for the
value of 0, and document accordingly.
Change-Id: I7c4d10ee50b4f79fe004761c62d24a4b1e5a750f
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/211071
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>