Add completions to field with Qt implementation

This is in response to a Google-Groups [discussion](https://groups.google.com/forum/#!topic/enaml/WMHkQOMifqg).  It adds `completion_list` and `completion_case_sensitive` declarative members to a `Field`, and implements them in Qt.  

I use the following code to test:

```python
from enaml.widgets.api import Window, Form, Label, Field

enamldef Main(Window):
    Form:
        Label:
            text = 'First Name'
        Field:
            completion_list = '''
Germany;Russia;France;
french fries;frizzy hair;fennel;fuzzball
frayed;fickle;Frobozz;fear;framing;frames
Franco-American;Frames;fancy;fire;frozen yogurt
football;fnord;foul;fowl;foo;bar;baz;quux
family;Fozzie Bear;flinch;fizzy;famous;fellow
friend;fog;foil;far;flower;flour;Florida
'''.replace('\n',';').split(";")
            completion_case_sensitive = True
```