Lazy retrieval of Java parameter names in completions.

When a completion is selected we need to show the parameter names as placeholders.
If the method comes from Java, we need to retrieve the Java element, a potentially
long-running operation that can trigger the structure builder. Instead of retrieving
all parameter names eagerly, for each completion proposal, we delay it to the
moment a completion is selected and inserted in the editor.

Fixed #1001560, #1001497.