Creating widgets tutorials

[jodator]

Suggested merge commit message (convention)

Docs: Two new tutorials.

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Additional information

NOTE TO SELF: Merge to stable.

TODO:

• Finish the tutorial about the block widget – missing a short section about adding a button.
• Review the "Creating the UI part" of the inline widget tutorial. The code samples and wording needs to be adjusted to the previous part. I changed many things when reviewing the first part (type => name, added the inspector, etc.) which changed the source code and wording. The last part needs to be adjusted to that and a screenshot needs to be made at the end.

[oleq]

Guess I'm done here.