[IMP] mail: improve usability of activities

This commit improves the daily use of activities through some small updates

 * on hover of clock icon in kanban card display tooltip. Purpose of this
   change is to clarify meaning of colors displayed in kanban view. Otherwise
   just havign a color is not very meaningful;
 * add days string in planned in column;
 * add space between number and days in planned in field;

This commit is related to task ID 1838315.

addons/mail/static/src/xml/web_kanban_activity.xml

@@ -4,7 +4,7 @@
 <t t-name="mail.KanbanActivity">
     <div class="o_kanban_inline_block dropdown o_kanban_selection o_mail_activity">
         <a class="dropdown-toggle o_activity_btn" data-toggle="dropdown">
-            <span class="fa fa-clock-o fa-lg fa-fw"/>
+            <span class="fa fa-clock-o fa-lg fa-fw" t-att-title="widget.selection[widget.activity_state]"/>
         </a>
         <ul class="dropdown-menu o_activity" role="menu">
         </ul>

addons/mail/views/mail_activity_views.xml

@@ -24,7 +24,7 @@
                             <label for="days"/>
                             <div>
                                 <field name="days" class="oe_inline"/>
-                                <span class="oe_inline">days</span>
+                                <span class="oe_inline ml4">days</span>
                             </div>
                         </group>
                         <group>
@@ -55,7 +55,7 @@
                 <field name="sequence" widget="handle"/>
                 <field name="name"/>
                 <field name="summary"/>
-                <field name="days"/>
+                <field name="days" string="Planned in (days)"/>
                 <field name="res_model_id" groups="base.group_no_one"/>
                 <field name="icon" groups="base.group_no_one"/>
             </tree>