tweak paddings in landcape mode (maximize space for preview & notes)

add minimal padding around slide preview (to not touch actionbar and
bottom of screen - horizontal whitespace will be larger becasue height
is limiting factor anyway)

avoid adding two left-paddings for the notes - the Text is "indented"
compared to the notes area already, so margin implied by the layout
weight distribution and the text indent is enough to separate it from
the preview.

avoid adding two right paddings for the note-text, as the text is not
justified and the word-break itself will account for a "padding" at the
edge, and the container already has a right-padding (that was reduced,
since the word-padding as well as centering in the unused space will
increase the effective margin)

Change-Id: I2a4605ba5a98eeed93a01db16d3d86c90df56470
Reviewed-on: https://gerrit.libreoffice.org/7230
Reviewed-by: Andrzej Hunt <andrzej.hunt@collabora.com>
Tested-by: Andrzej Hunt <andrzej.hunt@collabora.com>