[Evolution] Ambiguity between "highlighted" and "selected"

[Patrick O'Callaghan <poc usb ve>]

https://bugzilla.gnome.org/show_bug.cgi?id=760521

On switching to a message folder, the previously selected message in
that folder is highlighted, however the message is not in fact selected
until the user clicks on it, so that hitting an arrow key will jump to
the next folder in lexical order, not to the next message. However
hitting Reply *will* bring up the composer on the highlighted message.

The situation changes once the user clicks on the message because now
it is really and truly selected, but there is no visual change to
indicate this.

The potential confusion should be resolved either by making a visual
distinction or by defining the highlighted message to be selected
whenever it is highlighted.

poc