online-accounts: Set max width in chars for label

Set a limit of 40 chars to the label explaining the panel when no
accounts have been added yet. This prevents the label from expanding
itself and the whole window arbitrarily.

https://bugzilla.gnome.org/show_bug.cgi?id=720188

panels/online-accounts/online-accounts.ui

@@ -146,6 +146,7 @@
                         <property name="label" translatable="yes">Adding an account allows your applications to access it for documents, mail, contacts, calendar, chat and more.</property>
                         <property name="justify">center</property>
                         <property name="wrap">True</property>
+                        <property name="max_width_chars">40</property>
                         <style>
                           <class name="dim-label"/>
                         </style>