[IMP] mail, calendar: add today meetings in systray

Purpose of this commit is to display today meetings in the systray. This will
help people in their daily job to have quickly access to their meetings.
It is displayed along with activities and reminders to have all current work
to do in the same place.

Displayed meetings are ongoing or not yet started ones of the current day.
All-day meetings of the current day are also shown. Done meetings are not
displayed to lessen noise.

Clicking on the systray will open the calendar view of meetings in a day
mode. This is done by a small modification in the calendar view allowing
to choose the display mode from context in addition to the mode attribute
of the action definition.

This commit is linked to task ID 60593. Closes #24438. Thanks to @ged-odoo
for quick reviewing.