Resolves: tdf#122597 chart dialog should be modal

Change-Id: I66881168fe2c6f3d033f2037e3d1b884bb74bc1a
Reviewed-on: https://gerrit.libreoffice.org/66015
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

chart2/uiconfig/ui/attributedialog.ui

@@ -5,6 +5,7 @@
   <object class="GtkDialog" id="AttributeDialog">
     <property name="can_focus">False</property>
     <property name="border_width">6</property>
+    <property name="modal">True</property>
     <property name="default_width">0</property>
     <property name="default_height">0</property>
     <property name="type_hint">dialog</property>