Resolves: tdf#125132 xml signatures dialog should be modal

Change-Id: I6b8e93070e89e90341e02e8d714c47d4b0931845
Reviewed-on: https://gerrit.libreoffice.org/71899
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

xmlsecurity/uiconfig/ui/digitalsignaturesdialog.ui

@@ -24,6 +24,7 @@
     <property name="can_focus">False</property>
     <property name="border_width">6</property>
     <property name="title" translatable="yes" context="digitalsignaturesdialog|DigitalSignaturesDialog">Digital Signatures</property>
+    <property name="modal">True</property>
     <property name="type_hint">dialog</property>
     <child>
       <placeholder/>