Process input from user and update practice pane

The input from user can come in two forms:
  1. Target text typed by the user in the input textarea
  2. Clicking of links

When the user types the input, evaluate it and update the target
text element. If an error is found in the user's input, perform
the following tasks:
  1. Change the background colour of the target text box to
     pink.
  2. Change the text colour of the input typed by the user to
     red.
  3. Reset the colours to default once the user fixes the error.

If the user types the entire target text successfully, then
perform the following tasks:
  1. Change the background colour of the target text as well as
     that of the input textarea element to gray.
  2. Disable the input textarea element.

When the user clicks a different subunit link, perform the
following tasks after the new subunit is loaded:
  1. Clear the input textarea element.
  2. Reset the practice pane to the default state.
  3. Ensure that the target text displays the first character as
     the character to be typed by the user.