[gnome-2048] Add a valign=center.

[Arnaud B. <arnaudb src gnome org>]

commit 384e6a22ecf146691c4077befa772b1a72606781
Author: Arnaud Bonatti <arnaud bonatti gmail com>
Date:   Mon Feb 4 18:13:50 2019 +0100

    Add a valign=center.

 data/mainwindow.ui | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/data/mainwindow.ui b/data/mainwindow.ui
index e95972d..0ac51fb 100644
--- a/data/mainwindow.ui
+++ b/data/mainwindow.ui
@@ -36,6 +36,7 @@
             <!-- Translators: button in the headerbar (with a mnemonic that appears pressing Alt) -->
             <property name="label" translatable="yes">_New Game</property>
             <property name="visible">True</property>
+            <property name="valign">center</property>
             <property name="can-focus">True</property>
             <property name="receives-default">True</property>
             <property name="use-underline">True</property>