tdf#109100: Hide label for margin control in Impress

Change-Id: Ib61656913df673ebfde61e2c921f84cb81195a3b
Reviewed-on: https://gerrit.libreoffice.org/46974
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
(cherry picked from commit 88f6ffeb9e0c0b942c2b0bc9d60af7bb7a6caaf8)
Reviewed-on: https://gerrit.libreoffice.org/47119
Reviewed-by: Yousuf Philips <philipz85@hotmail.com>
diff --git a/sd/source/ui/sidebar/SlideBackground.cxx b/sd/source/ui/sidebar/SlideBackground.cxx
index 9c94a20..76da35d 100644
--- a/sd/source/ui/sidebar/SlideBackground.cxx
+++ b/sd/source/ui/sidebar/SlideBackground.cxx
@@ -240,7 +240,13 @@ void SlideBackground::HandleContextChange(
    if ( IsImpress() )
    {
        mpMasterLabel->SetText(SdResId(STR_MASTERSLIDE_LABEL));

        // margin selector is only for Draw
        mpMarginSelectBox->Hide();
        VclPtr<FixedText> pMarginLabel;
        get(pMarginLabel, "labelmargin");
        if ( pMarginLabel )
            pMarginLabel->Hide();

        if ( maContext == maImpressMasterContext )
        {