Initial autotools
Add polkit agent listener
Use polkit_agent_listener_register to fix compile warning

instead of deprecated polkit_agent_register_listener
Add initial dialog
Pack the widgets in a gtk table so they are aligned

Do it in a function so it will be easier to use GtkGrid when porting
to gtk3.
Connect show-info and show-error handlers for polkit session
Emit dialog response when pressing enter in password entry