This commit updates code to pass aria-label for icon buttons
in dropdown widget options so that they can be used in
dropdown_list.hbs template instead of directly defining them
in the template file.
We currently show buttons in saved snippets dropdown only
so defining the labels in template file worked correctly
but it is good to pass them as variables so that it is
simpler to add buttons for other dropdowns when required.
Renames "Add a new saved snippet" to "Create a new saved snippet" and
"Add saved snippet" tooltip to "Insert saved snippet".
Moves "Create a new saved snippet" button to the bottom sticky area of the
dropdown.
