Added the placeholder for UX images

This patch adds the placeholders into the image assets for UX element images to be used on the Main Library Window ToolBar.
Adds the slider with proper constraints

This patch adds the thumbnail size slider to the Main Library Window status line with the proper constraints to have it attached to the lower right end of the window.
Fixed method delegation error

This patch fixes an error found on the Main Library Window ToolBar, as it receives messages directed to the ToolBarDelegate protocol, which was implemented by another class, that is outside of the usual responder chain.
This is a temporary fix, as another, more elegant solution, is worked out.
Added project UX artwork files

This patch adds the project original UX related artwork files, for proper history keeping and maintenance.