Add ability to set and change a handle of a fieldset manually

[jmartsch]

Bug Description

When creating a new fieldset, the handle is automatically generated, based on the title.
It would be good, if we could choose and change the handle manually.

I set up my fieldset titles in german, but would like to have the handles in a view in english. Right now, this seems not to be possible.

How to Reproduce

Create a new fieldset

Environment

Statamic version: 3.0.0.-beta.26

PHP version: 7.3

**Install method (choose one):composer

• Starter kit