More precise labels for image and placeholder widgets

[Marek Lewandowski]

In issue #14539 we added getLabel method to the widget definition.

Now we'd like to utilize it in image2 and placeholder plugins.

I'd propose following labels:

• image: "<alt>" image widget
• placeholder: "<placeholderName>" placeholder widget

[Tomasz Jakut]

I've added getLabel method to placeholder and image2 plugins. Pushed changes to ​branch:t/14701.

[Marek Lewandowski]

1. Instead this.editor.lang.image2.pathName let's simply use this.pathName. It's shorter and allows devs to override it in case of need.
2. Image2: we're missing a space between alt and the path name, so we end up with aria-label="fooimage widget" instead of aria-label="foo image widget".
3. I'd love to see a single unit test for each plugin, so that we'll avoid regressions later on.

[Tomasz Jakut]

Added unit tests & fixed incorrect label for image2 plugin. Pushed changes to ​branch:t/14701.

[Marek Lewandowski]

R+ with minor change:

• Instead of calling this.wrapper.findOne( 'img' ).getAttribute( 'alt' ) for widgets we'd be better with this.data.alt. The former one would break if the widgets had added an img before the main img (e.g. drag handler were moved to the beginning of widget wrapper).

Added this change to ​t/14701.

[Marek Lewandowski]

Pushed to ​master at dev.