disable global paste action when clipboard is empty * main_window.cc(constructor): initialize class variable for the clipboard, (construct_edit_menu): don't set shortcut for paste action here, add new menu for clearing the clipboard, connect the clipboard's change signal to the new slot clipboard_has_changed, (clipboard_has_changed): new slot enabling or disabling the paste and clear clipboard action when clipboard has changed (clear_clipboard): new slot for the clear clipboard action * main_window.h: new slots clipboard-has_changed and clear_clipboard, new class variable _clipboard, new action _clear_clipboard