Strongly type the cell of NSTextField

Similar to what was done for NSButton and NSPopUpButton with https://github.com/mono/monomac/commit/501903ce97634578fdf4bf4408e6e51dde06bdfa.

This makes it simpler to set properties such as ```PlaceholderString``` programmatically.

Before this change, you'd have to manually cast NSTextField's Cell property to NSTextFieldCell like this:

```c#
((NSTextFieldCell)myTextField.Cell).PlaceholderString = "Search";
```

After this change, you can simply do this:

```c#
myTextField.Cell.PlaceholderString = "Search";
```