* src/controller.{cc,h} (Controller::edit_actions): Rename from
clip_actions.
(Controller::erase): Add action.
(Controller::load_xml): Fetch cut, copy, paste and delete menu
items from the glade file.

* src/mainwindow.{cc,h} (MainWindow::on_{cut,copy,paste,erase}):
Add and implement new signal handlers.
(MainWindow::connect_signals): Connect the cut, copy, paste and
erase actions to the new signal handlers.
(MainWindow::on_filetree_switch_buffer): Enable or disable the
edit actions based on the editability of the file buffer.