[gnome-2048] Make the Preferences dialog modal.

[Arnaud B. <arnaudb src gnome org>]

commit 17f8fd3b6e6a377ab14355b237aeef16a5ee6f6f
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Thu Jan 24 17:41:55 2019 +0100

    Make the Preferences dialog modal.
    
    Else, you can close it while playing,
    and then your game is restarted. Bad.

 data/preferences.ui | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/data/preferences.ui b/data/preferences.ui
index 3724153..e289486 100644
--- a/data/preferences.ui
+++ b/data/preferences.ui
@@ -33,6 +33,7 @@
     <property name="window-position">center-on-parent</property>
     <property name="type-hint">dialog</property>
     <property name="use-header-bar">1</property>
+    <property name="modal">True</property>
     <child internal-child="vbox">
       <object class="GtkBox">
         <property name="can-focus">False</property>