Undo resets Redo buffer when undoing to beginning

[damo]

The redo buffer is reset when using the undo feature all the way to the beginning.

To reproduce:

1. Open nightly Ajax sample
   
2. Add some text on a few lines
   
3. Use undo until reaching the initial state
   
4. Attempt to redo last undo
   

Result: Redo is enabled but does not have any more state

Expected: Redo should maintain state and allow redoing even after reaching the end of the undo stack

[garry.yao]

The problem was due to incorrect snapshot saving logic which create one unreachable snapshot at the beginning of stack.

[fredck]

• In the editingblock plugin, please fire the beforeModeUnload event at line 171. It may happen that the event is used to manipulate the data right before unloading it.

• Coding style: please make it correct and consistent with the rest of the code. Also, the change at line 95 is not needed.

[fredck]

Sometimes words are not enough to properly express your intentions. I've attached a new patch for it. Please go ahead committing it if it satisfy the ticket needs.

[garry.yao]

Fixed with [3846] and [3847].