Limit "should see a button" to actually visible elements
Less assumptions in "HTML field contains" step

Not using the loose idea of a "field name" in the NamedSelector
sense of the word as "field with id exists".
Behat: More robust "field should contain" logic
Behat: "Insert image from web" feature

Partially fixes https://github.com/silverstripe/silverstripe-cms/issues/628
Modified a bit by Ingo :)