Added a phone number watermarks on the registration pages.

* New WatermarkedTextBox widget that extends TextBox to support
  this function.
* Added in different watermarks for each individual country as per
  the phone number format for each country.
* NOTE: One must apply a cheap hack of calling .setText("") when
  changing the watermarks using event handlers due to some issues
  with the styling. An example of this can be found on RegisterScreen
  line 96.