Add dbus support for controlling the notes

[collinss]

• Adds a dbus service file to allow the notes to be activated even when closed. (build switched to meson to make it easier to install it to the correct location.)
• Adds the following dbus methods and signals:
  • ShowNotes (method): toggles visibility and focus of the notes on the screen
  • NewNote (method): creates a new note containing the provided text
  • NewNoteBlank (method): creates a new empty note
  • NotesChanged (signal): indicates that the notes have changed in some way and the changes have been saved