Add Prepend method to Accordion

[JeffreyLeeLi]

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?

I'm frustrated to manipulate Items field directly, 'coz it'll panic.

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

None works for me now.

Describe the solution you'd like to see.

func (a *Accordion) Prepend(item *AccordionItem)

[Jacalz]

I'm not necessarily saying that I'm against adding this, but I want to clarify that "coz it'll panic" is not a good enough reason for adding this. As explained in the previous issue, it panics because of a race condition that that has very likely been fixed on the develop branch for 2.6.

[andydotxyz]

This sounds like a reasonable addition - other widgets with Append have added Prepend too

[JeffreyLeeLi]

Thanks for the adoption.

[andydotxyz]

Added on develop branch ready for 2.6.0 release