tdf#139965: Broken master slide list after switching mode.

Let's update the master slide combobox by context change.
populateMasterSlideDropdown() will fill the list when we
switch to normal view. The same method is called when the
sidebar created in normal view.
mpMasterSlide->Clear() will clean the list when we switch
to master view. The same happens, when the sidebar is created
in a master view context (the list is empty).

Change-Id: I322619e409a5352ddcd59a249dd0c874054c1e4c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/110075
Tested-by: Jenkins CollaboraOffice <jenkinscollaboraoffice@gmail.com>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
(cherry picked from commit 2be52f741e283b5f172ae536f0e99ce0f38df9e1)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/110114
Tested-by: Jenkins
diff --git a/sd/source/ui/sidebar/SlideBackground.cxx b/sd/source/ui/sidebar/SlideBackground.cxx
index e92c936..17c85e2 100644
--- a/sd/source/ui/sidebar/SlideBackground.cxx
+++ b/sd/source/ui/sidebar/SlideBackground.cxx
@@ -301,6 +301,7 @@ void SlideBackground::HandleContextChange(
            mxCloseMaster->show();
            mxEditMaster->hide();
            mxMasterSlide->set_sensitive(false);
            mxMasterSlide->clear();
            mxDspMasterBackground->set_sensitive(false);
            mxDspMasterObjects->set_sensitive(false);
            mxFillStyle->hide();
@@ -312,6 +313,7 @@ void SlideBackground::HandleContextChange(
            mxCloseMaster->hide();
            mxEditMaster->hide();
            mxMasterSlide->set_sensitive(false);
            mxMasterSlide->clear();
            mxDspMasterBackground->set_sensitive(false);
            mxDspMasterObjects->set_sensitive(false);
            mxFillStyle->hide();
@@ -323,6 +325,7 @@ void SlideBackground::HandleContextChange(
            mxCloseMaster->hide();
            mxEditMaster->show();
            mxMasterSlide->set_sensitive(true);
            populateMasterSlideDropdown();
            mxDspMasterBackground->set_sensitive(true);
            mxDspMasterObjects->set_sensitive(true);
            mxFillStyle->show();