Split displayTargetText into two functions

Split the displayTargetText function into two functions:
  1. setTargetText
  2. displayTargetText

While the setTargetText function does most of the tasks that
displayTargetText was doing earlier, i.e. determining the target
prefix, target character and target suffix, it doesn't display
the target text anymore. Instead, it sets these strings into the
tutor object.

The displayTargetText function combines these strings into a
target text string, styles the target character and displays it
in the target pane.

While making this change, perform the following refactoring
tasks:
  1. Add more properties to the tutor object to represent the
     current state of the tutor, so that the displayTargetText
     function can work on the data set by setTargetText
     function.
  2. Modify the setSubunit function so that it sets the current unit
     number, the subunit titles for the current unit as well as
     the current subunit text in the tutor object.
  3. Remove the call to the displayTargetText function in the
     setSubunit function.
  4. Remove the code to determine the subunit titles for the
     current unit from the displaySubunitLinks function because
     this is now taken care of in the setSubunit function.