[Version Control] General fixes to Checkout Command:

ProgressDialog now exposes if it's cancelled or not.
Set the MessageDialogProgressMonitor as slave so we can access its IsCanceled.

[Git] Implemented Cancelling of Checkout.
[Version Control] Win32 Subversion Cancel for Checkout.
[Version Control] Implemented cancelling for Unix Subversion
[Version Control] Refactor cancelling by using an event.
Added catch for cancelling the operation
Return IntPtr.Zero now
Merge pull request #386 from mono/vcsCancel

[Version Control] Implemented support for cancelling checkout.