[MainUI] Add feature to clear configuration parameters

[lsiepel]

While working on openhab/openhab-core#5238 we noticed that sometimes default values are materialized and we are unable to clear the value in the UI.

Some parameters like a string have a "X" next to the input field to clear the value. Otehrs like a stepper or other input type does not have such an "X" (or whatever button) to clear/unset the value.

[jimtng]

I've recently installed Discourse for my own use, and it has a very nice system to deal with this:

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This shows how the "default values" (Text, or a list of options) that haven't been overridden look like

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Then when I typed something to customize it

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And when I applied that change, it shows it's not default

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But if I typed something that is equal to the default, it reverts back to "not modified"

And this is not the behaviour that we want, right? We want to be able to distinguish the "default" (never touched) vs - someone specifically wanting to override the default and store it even though it is actually the same as the default.

[jimtng]

On the topic of stepper input:

Once applied:

So it's not using an obscure [x] - it uses a prominent / clear "reset" button, separate to the typical [x] that we sometimes have for text inputs. So regardless of input type it can always display a consistent reset button.

Here's one for a drop down input:

And here's one for an array entry:

[lsiepel]

This would be very nice to have in MainUI!

[jimtng]

On mobile screen, the reset buttons appear after the description