Add support for autocomplete for object initialization fields

[colinkiama]

For example, while writing this code block:

var label = new Gtk.Label () {
    text = "Hello World"
};

Gtk.Label field suggestions should be displayed while writing the line for the text field.

[Prince781]

This should hopefully be less complicated to achieve now that Vala.MemberInitializer is a Vala.Expression (wasn't true the last time I checked).

[colinkiama]

I'm sorry, this is a duplicate of #242. I just noticed now