Merged Hide Whitespace with Browser Mode and hooked
the former with the UI. In Hide Whitespace mode
the headers and footers are removed and
page margins as well as between-page gaps are
reduced to minimum.

In single-page view the side margins are left
as-is. In multi-page and book-view the sides
are removed, however, they have twice the
width of the top and bottom.

Printing and PDF export disable this mode.

The usable screen real-estate is greatly
improved in this mode, which is controlled
from the View menu.

Change-Id: I2d0075ae9a76162c26c78e9eb23a71920cdfc522
Reviewed-on: https://gerrit.libreoffice.org/17281Reviewed-by: Jean-Baptiste Faure <jbfaure@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit 2b8e62f7)