Add more experiment Web mockups
new file:   web/experiments/headerbar-only-notabs/00 - Editable.xcf
new file:   web/experiments/headerbar-only-notabs/01 - Pages View.png
new file:   web/experiments/headerbar-only-notabs/02 - Normal View.png
new file:   web/experiments/headerbar-only-notabs/03 - Cog Menu.png