[IMP] link_tracker: add placeholders

Add placeholders to the mediums and sources tree views.

Task-3240966

Part-of: odoo/odoo#116230

addons/utm/views/utm_medium_views.xml

@@ -5,7 +5,7 @@
         <field name="model">utm.medium</field>
         <field name="arch" type="xml">
             <tree string="Mediums" editable="bottom" sample="1">
-                <field name="name"/>
+                <field name="name" placeholder='e.g. "Email"'/>
                 <field name="active" invisible="1"/>
             </tree>
         </field>

addons/utm/views/utm_source_views.xml

@@ -5,7 +5,7 @@
         <field name="model">utm.source</field>
         <field name="arch" type="xml">
             <tree string="Source" editable="bottom" sample="1">
-                <field name="name"/>
+                <field name="name" placeholder='e.g. "Christmas Mailing"'/>
             </tree>
         </field>
     </record>