Add support for Parse on OSX

This patch adds support for Parse cloud storage and hosting service for OSX. This is a preliminary support, and is known to be broken. There seems to be a problem with the -F<framework_path> option of the linker (ld).
Removal of source DropboxSDK framework

This path removes the DropboxSDK framework source files stored there by mistake.
Add Slider to Library Window

This patch adds a Slider to the Library Window to control the size of the eBooks thumbnails presented to the user. It isn't yet functional, just a simple exploration of UX layout.