lok: avoid emission storms of un-necessary invalidations.
Common when constructing widgets with VclBuilder - which avoids
the 'Show' detection Pranav introduced in
8de98e61fbc.
This saves ~80% of the ~100k mostly bogus calls I get to:
desktop::CallbackFlushHandler::processWindowEvent when opening
and closing a few windows.
Change-Id: Ie508d6e19274472b85543275aee33f078ddcbbb2
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/86534
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>