Highlight image url portion when clicking image button

[jleclanche]

Clicking the image button currently looks like this:

I suggest changing it to behave this way:

When inserting an image it's most likely that the user will insert the image's url immediately, rather than the (optional) alt text.

Might be beneficial to change image-url to something like https://example.com/your-image.png instead, as well.

[andrerpena]

Right. I will do it now