tdf#91920 sw page gutter margin, from top: add UI

tdf#91920 sw page gutter margin, from top: add UI

- read from sw::DocumentSettingManager::mbGutterAtTop

- improve the preview widget to take gutter position into account

- also change it when the gutter position widget changes

- write it back to sw::DocumentSettingManager::mbGutterAtTop on
  modification

- always check if the dialog's input item set has SID_ATTR_CHAR_GRABBAG,
  otherwise do nothing to keep Calc/Impress unchanged

[ And in this branch, fix a silent merge conflict that mis-positioned
labelGutterMargin in the .ui file. ]

(cherry picked from commit 63d3bf99101e2138a21a74a3d717d4f25d3d242f)

Conflicts:
	cui/uiconfig/ui/pageformatpage.ui
	sw/source/core/doc/DocumentSettingManager.cxx

Change-Id: I05ef864bf7ac18f4c6829d78312044560e6402d0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/110671
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>