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/devices/480x854/res/ui.xml b/gui/devices/480x854/res/ui.xml
index dd7efd7..c4ce65f 100644
--- a/gui/devices/480x854/res/ui.xml
+++ b/gui/devices/480x854/res/ui.xml
@@ -50,6 +50,7 @@
 		<variable name="col1_x" value="10" />
 		<variable name="col2_x" value="240" />
 		<variable name="col_center_x" value="128" />
+		<variable name="col_center_medium_x" value="183" />
 		<variable name="center_x" value="240" />
 		<variable name="row1_y" value="140" />
 		<variable name="row2_y" value="290" />