Provide more user-friendly behaviours when the user presses the 'back' key

[GoogleCodeExporter]

The 'Back' key currently quits the application when the user is navigating 
through the files. This was surprisingly easy to do and may lead to a poor 
user experience. Please can we add a 'confirm exit' dialog before the 
application quits?

What steps will reproduce the problem?
1. Start the Daisy Reader
2. Navigate through the folders on the SD Card in the UI. Go into a folder
3. Press the back key

What is the expected output? 
- Return to the parent folder / directory
- If the user is at the top-level folder - ask them if they would like to 
exit the application

What do you see instead?
- The application quits without warning or a message.

Original issue reported on code.google.com by julianharty on 27 Nov 2009 at 6:35

[GoogleCodeExporter]

Various changes, up to and including r147 have hopefully improved the overall 
user experience e.g. adding the home screen, and the changes in r147 
http://code.google.com/p/android-daisy-epub-reader/source/detail?r=147 .

Original comment by julianharty on 5 Jul 2010 at 1:37

• Changed state: Fixed