Show current status in tooltip

### Problem

Because of https://github.com/elementary/wingpanel-indicator-datetime/pull/294 we are now able to Middle-click to open the Calendar (thanks @danrabbit!). To make this shortcut discoverable, we added a tooltip.

What's still missing however, is displaying the current state. I feel this is missing, because all the other tooltips in Wingpanel do show some sort of current state; for example the Volume tooltip:


### Proposal

We could add some text, which summarizes the plans for the current day. Something like:

**1 event and 3 tasks planned for today**
_Middle-click to open Calendar_

Other states:

- If there are events but no tasks:
- 2 events planned for today
- if there are tasks but no events:
- 1 task planned for today
- if there are no events and no tasks:
- Nothing planned for today

### Prior Art (Optional)

See other tooltips in Wingpanel.