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.
* Change RadioGroup orientation to vertical, as per discussion with
  Design Team.
* Removed hardcoded string.
* This class needs a lot more love.
Fix testcases that used hardcoded String
Fix tests related to colors and Button state
renamed clickable to enabled
set button final
Merge pull request #253 from Catrobat/NewVariableDialogFix

Refactor NewVariableDialog. Still a mess!