desktop: Add the capability to copy / paste dive number and date / time.

This is adding the capability to select 'Dive number' and 'Date / Time'
in the 'Copy dive components' dialog, and then copy them into the
clipboard.
When using 'Paste dive components, these values will then be pasted into
the selected dive(s).
This is intended to help with workflows that import dive information
from two different sources, like general information from another
logging program, and CCR ppO2 sensor readings from a unit log, and then
stitch them together into one cohesive entry with all data per dive.
Copied data is also output into formatted text when pasting the
clipboard outside of the application:

```
Dive number: 401
Date / time: Sun 2 May 2021 12:00 AM
```

No translations have been added as of now - I could not find any
information on how strings are translated for this project.

Signed-off-by: Michael Keller <github@ike.ch>

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