Because notifications to all views were disabled
(to avoid moving the cursors of everyone when someone
adds a comment) invalidation was also broken. The
issue was observed when a comment crossed tile
boundaries, the other tile was not redrawn.

This forces invalidation so tiles are rendered
correctly, fixing this regression.

One small issue remains in that invalidating a
comment only invalidates the text-area and not the
complete PostIt (specifically, the author and date
are not invalidated). This means that they can
still get trimmed when crossing tile boundaries.
That will be addressed separately, as it was an
existing issue before the regression this patch
fixes.

Change-Id: Ic8173e4873d58836883b897f735fc627bb0572ee
Reviewed-on: https://gerrit.libreoffice.org/31091Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>
Tested-by: Ashod Nakashian <ashnakash@gmail.com>