[MERGE][IMP] mail, note: improve activity summary view and use

Main specifications
 * set mail as dependency on contacts as activity view is used in contact
   that does not depend on mail -> crash;
 * update icon on the reminders: use calendar instead of clock;
 * add an icon to Contact in the systray;
 * add the Activity icon in the upper right corner of the Contact;
 * sort Activities by date: from more urgent to less urgent;
 * on the contact name in the activity view, the name is clickable but the
   cursor is wrong: use the little hand as a cursor;

Please see sub-commits for more details. Thanks to @alexkuhn for
reviewing and @est-odoo for testing.

This merge is linked to task ID 1882545 and closes PR #26797.

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