Combine fieldset property adding with the add property combobox

[johno]

It's a bit clunky to have two different comboboxes. Currently we have them for:

1. Adding a top-level property to a node
2. Adding a fieldset (pseudo element or class)

Ideally, we can combine the fieldset entry with the property box with a flow similar to:

Initial state

Typing a pseudo name (with or without preceding :)

Selecting a pseudo from the filtered dropdown

Displaying that selection (and making it dismissible with DEL)

ENTER is hit

This would add to PSEUDO_NAME.PROPERTY_NAME in the style object and clear the input back to the initial state