tdf#133426 Make mail merge progress dialog modal

This prevents doing things like trying to start another mail
merge for the same doc in parallel, thus crashing LibreOffice.

Change-Id: I5df2e804d97e82cccca4f0e297cb6d3396652882
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/94933
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
diff --git a/sw/uiconfig/swriter/ui/mmcreatingdialog.ui b/sw/uiconfig/swriter/ui/mmcreatingdialog.ui
index c7c787c..3af13d3 100644
--- a/sw/uiconfig/swriter/ui/mmcreatingdialog.ui
+++ b/sw/uiconfig/swriter/ui/mmcreatingdialog.ui
@@ -5,6 +5,7 @@
    <property name="can_focus">False</property>
    <property name="border_width">6</property>
    <property name="title" translatable="yes" context="mmcreatingdialog|MMCreatingDialog">Mail Merge</property>
    <property name="modal">True</property>
    <property name="type_hint">dialog</property>
    <child internal-child="vbox">
      <object class="GtkBox" id="dialog-vbox1">