Allow multiline field to update text from a thread

As it stands, the text attribute of a `MultilineField` cannot be updated outside of the MainThread.  The following code throws the bug:

```
QObject: Cannot create children for a parent that is in a different thread.
(Parent is QTextDocument(0x1e57f30), parent's thread is QThread(0x19d2598), current thread is QThread(0x1e62770)
QObject::connect: Cannot queue arguments of type 'QTextCursor'
(Make sure 'QTextCursor' is registered using qRegisterMetaType().)
QObject: Cannot create children for a parent that is in a different thread.
(Parent is QTextDocument(0x1e57f30), parent's thread is QThread(0x19d2598), current thread is QThread(0x1e62770)
```

```python
import threading
import time
from enaml.widgets.api import *
from atom.api import Atom, Unicode


class Test(Atom):
    
    foo = Unicode()
    
    def foo_updater(self):
        while 1:
            self.foo += 'hello\n'
            time.sleep(1.)


enamldef Main(MainWindow):
    attr test = Test()
    attr thread = None
    activated ::
        thread = threading.Thread(target = test.foo_updater)
        thread.setDaemon(True)
        thread.start()
    Container:
        Field:
            text << test.foo
        MultilineField:
            text << test.foo
```

After the patch, the code runs just fine.