Refactor NewVariableDialog. Still a mess!

* Moved all Dialog logic into its own `.java` file
* Removed code that manually changed `Button` text color and `EditText`
  background color.
Code like
```Java
positiveButton.setClickable(false);
positiveButton.setTextColor(getResources().getColorStateList(R.color.gray));
```
has been replaced by
```Java
positiveButton.setEnabled(false);
```
where seen apt.

* Changed `RadioGroup` orientation to vertical, as per discussion with
  Design Team.
* Removed hardcoded string.

**These classes need a lot more love!**