AppWindow: Change file.close to win.close.

* glom/application.[h|cc]: Add the close action to the standard
  appwindow menu group, instead of our custom file group.
* ui/operator/window_main.glade: Rename it here too.

  There is no reason not to do this, though I still do not know
  of any advantage to it.
Because there is no need not to.
Application: Manually specify accelerators.

* glom/applicaiton.[h|cc]: Use add_accelerator() to make the
  accelerators (already specified in the window_main.glade file)
  actually work.
* glom/appwindow.cc: Add a comment about it.

  See GTK+ bug 708905