Some bug fixes in Input + add title to App constructor

[darrenburns]

• Adds argument to App allowing you to set App.title via the constructor
• Two Input.Changed events were being sent on initialisation of Input widget. This event will now only be sent if a non-None value is set using the constructor. (Signature of Input init changed slightly - value is now Optional.)
• The validator for view_position was being called on every re-render (self.view_position = self.view_position). I'm not sure why this was the case - but the validator is immediately called anyway via watch_cursor_position. Is there some situation where view position can change NOT as a result of cursor position that I'm not thinking of?