Converted Tiff reads to use tifffile module

Following problems installing pylibtiff on other platforms
Removed NXroot from the Add Data menu options

New root items should be added with the New File menu item.
Fixed bug with MainWindow.close

It required two arguments before (a legacy of the iPython QtConsole),
but only one was sent as a trigger. This wasn't a problem on Macs,
since the Quit command isn't added to the File menu and so used a
different action. I've also added some new keyboard shortcuts.
Added explanatory text to tree add

Makes it clear where a non-NXroot group ends up.
Fixed bug with copying data from one file to another
Added .hdf5 to list of extensions
Added unlock icon

To show when the root item is stored in a file that is unlocked