Major refactoring in preparation for photo editing.  New Photo object centralizes pixbuf generation and photo manipulation, and introduces GObject signals into the system so various interested parties (esp. various pages) are notified when a photo is modified or removed.  Also refactored Dimensions struct, as what started out as a utility record has become a first-class citizen of Shotwell.  Also, while refactoring, went ahead and made changes to reflect Yorba's coding style (i.e. camelCase for variables -> underscored_names).  In the process, knocked out #198: Databases and thumbnail cache always in sync with modifications.  #202: Black-and-white shading with crop tool.