Add some explanation text to top of widgets

[dstansby]

This PR adds some text to the top of widgets explaining how many layers it takes as input. I'm hoping this improves user experience somewhat.

The name of the widget should probably have some title styling applied to it - @samcunliffe can you advise on the best way to do this? If you don't know off the top of your head I can look into it.

Fixes #27

[samcunliffe]

@samcunliffe can you advise on the best way to do this? If you don't know off the top of your head I can look into it.

Off the top of my head: not, I'm afraid. I can tell you button sizes in the CSS or colours. Text formatting is advanced stuff. From a quick cmd-F search of the CSS it looks like you could try and use <h1> inside the QLabel, or put it in a QCustomTitleBar (or something similar) and it should just work™️.

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Worth noting that Napari already tells you the widget name (albeit quite faintly)

[samcunliffe]

🚀 Almost fixes

• Add contextual GUI hints on using widgets #27

Do you know how to do my wishlist for that? (Make the help hints disappear when the user has selected the required number of layers.)

That said, a static QLabel is simpler (and therefore probably better).

[dstansby]

To keep this simple (partly because there's already the widget name as you pointed out), I've updated it to just add the text for how many layers need selecting.