Add cancel backup capability.

This will stop the iteration of the partition objects, kill the
current twrpTar thread and remove the backup directory.

Implement TWAtomicInt class to give us a wrapper that automatically
uses mutexes before the read and write to help ensure that the
reads and writes will be atomic based on documentation.

Change-Id: I645b22bc980a292e9c7202acb24ffd22ebe68c63
diff --git a/gui/objects.hpp b/gui/objects.hpp
index 99b5855..832569c 100644
--- a/gui/objects.hpp
+++ b/gui/objects.hpp
@@ -360,6 +360,7 @@
 	int startmtp(std::string arg);
 	int stopmtp(std::string arg);
 	int flashimage(std::string arg);
+	int cancelbackup(std::string arg);
 
 	int simulate;
 };