mailmerge: Make the SwMailMergeConfigItem instance long-living.

In order for the toolbar to work, we need the SwMailMergeConfigItem instance
in the view living even when the mail merge wizard is not around.

With this change, there's a small glitch that the SwMailMergeChildWindow
sometimes stays open even when we leave the wizard, but I plan to remove this
window / button anyway, so no real harm at the moment.

Change-Id: I2bf0c49504b9cf2f17f6fce7ab4cab7cb5be6426

sw/source/ui/dbui/mmoutputpage.cxx

@@ -906,7 +906,6 @@ IMPL_LINK_NOARG_TYPED(SwMailMergeOutputPage, PrintHdl_Impl, Button*, void)
     pTargetView->ExecPrint( aProps, false, true );
     SfxGetpApp()->NotifyEvent(SfxEventHint(SW_EVENT_MAIL_MERGE_END, SwDocShell::GetEventName(STR_SW_EVENT_MAIL_MERGE_END), pObjSh));
 
-    pTargetView->SetMailMergeConfigItem(nullptr, 0, false);
     m_pWizard->enableButtons(WizardButtonFlags::CANCEL, true);
     m_pWizard->enableButtons(WizardButtonFlags::FINISH, true);
 }