Fixed an issue where clicking the selected row in the autocomplete table would clear the autocomplete text view.

Instead of using the PLTableViewDidReceiveMouseDown notification (which is no longer posted in this commit by PLTableView), use a combination of setAction: and tableView:shouldSelect: row to determine if rows are selected in the table view and display the autocomplete text view.
Cells in the autocomplete table view are no longer editable.

Implemented the tableView:shouldEditTableColumn:row: method and return NO.
Display a "No Completions" message when no completions are available and the autocomplete display is visible.

Added a subview to the autocompleteScrollView that is a non-selectable NSTextField displaying the above message.