Set `incline=<Number>` via level

[tordans]

This is a follow up to https://github.com/bryceco/GoMap/issues/189#issue-430492964. I might have a use case now, that is important enough to justify a feature for this:

Playground slides and steps.

We started micromapping playground in our area of Neukölln. Having the incline of a slide is an interesting data attribute. Once can find the steepest slide this way :-).

Example: https://www.openstreetmap.org/way/908479258

@SupaplexOSM started visualizing playgrounds in detail in his rendering at https://supaplexosm.github.io/strassenraumkarte-neukoelln/?map=micromap#20/52.47469/13.44315

The idea: Add a (+) button on every key that holds incline which shows a panel similar to the direction panel. It uses the iPhone level feature to set the level.

Image source https://support.apple.com/en-gb/guide/iphone/iphbd435673d/ios

Ideally, the app would add the unit ° or % automatically. And respect the direction of the line, if a slide is mapped as a line (negative numbers meaning down following the direction of the line).

In https://github.com/bryceco/GoMap/issues/189#issuecomment-486016508 Bryce rightfully asked for numbers. As expected they are not high. The key incline together with playground is used 175 times: https://taginfo.openstreetmap.org/keys/playground#combinations. And until now, incline in general is used mostly with up|down as value, not numbers (https://taginfo.openstreetmap.org/keys/incline#values).