Add support for "classification" edit widget

[nyalldawson]

While it's not a highly used edit widget type, it's still quite convenient for quickly setting up a widget which matches the categories you've already setup for a layer's renderer. I've seen a few users use it as a result...

Reproduction Steps:

1. [First Step]
2. [Second Step]
3. [Other Steps...]

Expected behavior:

[Describe expected behavior here]

Observed behavior:

[Describe observed behavior here]

Screenshots and GIFs

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QField version: [Enter QField version here including build number found in the about screen after the release name example 1.2.0 (10200993)]

Additional information:

• Problem started happening recently, didn't happen in an older version of QField: [Yes/No]
• Problem can be reliably reproduced, doesn't happen randomly: [Yes/No]
• Problem happens with all files and projects, not only some files or projects: [Yes/No]

[romainbh]

Because classification is often easier and faster than drop-down lists, it would be great if this function were supported by QField ;-)