Selection highlighting in the calendar application

[ZerkerEOD]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

When selecting a date from the calendar widget, the selection doesn't appear to show on the calendar (ie. highlight the date selected)

Is it possible to construct a solution with the existing API?

Sounds like it, per our discussion on Discord, you mentioned that the term 'selection' exists in the api and theme.

Describe the solution you'd like to see.

Ideally, a way to use the API/Theme of the calendar widget to highlight the selected date when on a single date picker, or a way to programmatically to highlight dates between multiple selected dates. Not sure if the selection will take a range or not.

[Jacalz]

The Calendar widget has not yet moved into the main Fyne repository but is part of fyne-x. I'll move the issue there.