DataEntryForm: refactored displayInitialDialog method

- Use a string instead of a JLabel object for the message in the displayInitialDialog method.
- Removed the html tags since these are not needed for a plain text message.
- Show the absolute root directory to make the message even more informative (and increased the width of the dialog).
- First pack the initial dialog and then center it on screen (otherwise the width and height are still zero).
- Added Javadocs for the initialDialog and rootDirectoryName fields and terminated some lines in existing Javadocs with a period (Checkstyle).
- Removed two unused fields: inputText and message1.