[fractal] Add Shift+Enter shortcut in shortcuts window

[Julian Sparber <jsparber src gnome org>]

commit 62f076a1b8bf9362979450ec827db147de9aa222
Author: Giovanni Grieco <giovanni grc96 gmail com>
Date:   Sat Dec 15 16:43:18 2018 +0100

    Add Shift+Enter shortcut in shortcuts window
    
    This fixes issue #401

 fractal-gtk/res/gtk/help-overlay.ui | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/fractal-gtk/res/gtk/help-overlay.ui b/fractal-gtk/res/gtk/help-overlay.ui
index 7774fa3..2b49052 100644
--- a/fractal-gtk/res/gtk/help-overlay.ui
+++ b/fractal-gtk/res/gtk/help-overlay.ui
@@ -41,6 +41,19 @@
             </child>
           </object>
         </child>
+        <child>
+          <object class="GtkShortcutsGroup">
+            <property name="visible">true</property>
+            <property name="title" translatable="yes" context="shortcut message">Composing a new 
message</property>
+            <child>
+              <object class="GtkShortcutsShortcut">
+                <property name="visible">true</property>
+                <property name="accelerator">&lt;Shift&gt;Return</property>
+                <property name="title" translatable="yes" context="shortcut message">Write on a new 
line</property>
+              </object>
+            </child>
+          </object>
+        </child>
       </object>
     </child>
   </object>