[Shotwell] Source code reorganization

[Jim Nelson]

Yes, I'm hoping the new system will put parent and derived classes together
when it makes sense.  Sometimes it might not -- for example, Photo and Video
derive from MediaSource, but my thinking is to put photo-related code into a
photo unit and video-related code into a video unit.

  For example the PhotoCanvas class is inside
> EditingTools.vala when the classes inside EditingTools only have it as
> an instance variable and are not really related in any other way to
> it.
>
>
The PhotoCanvas class is a mediator class between the editing tools and the
EditingHostPage.  Although it's only an instance variable for them, it's a
pretty important instance variable, as it's how the draw to the screen and
receive notifications of user events.

-- Jim