Allow in preview diff screens to select patch sets to be compared

Similar to the normal diff screens the preview diff screens have now a
header that shows the patch sets so that the user can change the
patch sets that are compared.

Change-Id: Ibb9d87ef8ee301170586f2b4b533ca086a04144d
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>