Problem:

1. (a) copied April
2. (a) pasted -> April
3. (b) copied March
4. (a) pasted -> March [should have been April]

where (a), (b) are different views

Solution:
A real solution would require to have one clipboard per view.
This patch is only a workaround, which doesn't allow to paste content
which has been copied in a different view; it takes also care to
disable the "Paste" entry in the context menu.

Change-Id: I3254f130af106299b0b519884a4ca03db08fc4c8
Reviewed-on: https://gerrit.libreoffice.org/40459Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan Holesovsky <kendy@collabora.com>