Refactor code to add inspection dialog.

Refactor code to add inspection dialog.

Turn Filetree into a Gtk object.

This changes the public interface of Filetree to be a Gtk object,
inheriting from GTree.view.  This allows us to attach signals to this
object.

Second, we add the inspection dialog as a separate module.

The filetree and inspection dialog modules are loosely coupled by two
signals.  The tree#op_inspection_dialog signal is emitted by the tree
when the inspection dialog should be opened.  tree#clear_tree is
emitted by the tree when the inspection dialog should be closed (it
can also be closed by user action).

The inspection dialog is not implemented by this commit.  It is just
empty for now.

The other operations are modified to work in the same way as the
inspection dialog.  eg. file checksum is implemented using a separate,
loosely-coupled module called Op_checksum_file, which listens for a
tree#op_checksum_file signal and runs when that signal is emitted.