Added scalebar option to ol.control.scaleline

[weskamm]

This adds an option to show scalebars instead of a scaleline in the ScaleLine control.

Example and API doc have been adjusted.

[tschaub]

Thanks for the contribution @weskamm! My suggestions above are mostly about shorter option names.

I'm curious what you and others think about making the default style look a bit more like the current scaleline style.

Also wondering if it is possible to keep the bar shorter for longer.

[ahocevar]

Thanks @weskamm. I share @tschaub's concerns about the length of the scale bar. Regarding the style, I also agree that it should look a bit more like the style of the other controls. However, we could also think this the other way around and make the default theme for other controls look more black/white/grey.

[ThomasG77]

I'm not sure to understand the purpose of this PR as "ol-ext" has already an extension for this e.g http://viglino.github.io/ol-ext/examples/canvas/map.canvas.control.html
Should it be maintain in the core? I misunderstood?

[tschaub]

However, we could also think this the other way around and make the default theme for other controls look more black/white/grey.

I like that idea. Probably best for a separate PR.

[marcjansen]

A very nice PR, and a nice set of comments / suggestions.

I am 👍 on merging, once the remarks are adressed.

Thx.

[weskamm]

Thanks for the reviews!
I adressed the issues mentioned by @tschaub (renamed props, fixed example, localization).

Regarding the length of the scalebar: I use a non-default minWidth in the example, because the default width of 64 pixels is a bit small for a scalebar (but ok for scaleline). Thats why the sizes differ. I just reduced the width in the example to get more similar results

Regarding the style: I would like to keep the current black and white look, as this is the way a scalebar typically looks like.

[tschaub]

Thanks for the great contribution @weskamm.