Renders a dropdown HTML element (select).

'name' - the name of html control. The name is attributized before
being rendered.

'selections' - a list of strings to render as selections. Each element
may be a string or a cons cell. If it is a cons cell, the car of each
cell will be used as the text for each option and the cdr will be used
for the value.

'id' - an id of the element.

'class' - css class of the element.

'selected-value' - a list of strings. Each option will be tested
against this list and if an option is a member it will be marked as
selected. A single string can also be provided.

'welcome-name' - a string used to specify dropdown welcome option (see
*dropdown-welcome-message*). If nil, no welcome message is used. If
'welcome-name' is a cons cell, car will be treated as the welcome name
and cdr will be returned as value in case it's selected.