fdo#73151 Make better use of the sidebar

The general idea is to encourage use of the sidebar instead of floating windows (for Navigator, Styles&Formatting and Gallery)

Changes:
* Show the sidebar by default in Writer & Impress
* Remove the Gallery floating window (Gallery now always opens in the sidebar)
* Remove all Gallery, Navigator and Styles&Formatting links from the default toolbar in Writer
  (since they have an icon in the sidebar and the sidebar is shown by default now)
* When selecting "More" from the Styles dropdown, the Stylelist opens in the sidebar instead of the floating window

This has all been discussed extensively here: https://bugs.freedesktop.org/show_bug.cgi?id=73151

Change-Id: I3a0461d1472711da7121801000af294b432fccb1
Reviewed-on: https://gerrit.libreoffice.org/11077
Reviewed-by: Thomas Arnhold <thomas@arnhold.org>
Tested-by: Thomas Arnhold <thomas@arnhold.org>