Bounding box tests exclude higher bound, so touching items (zero spacing) don't report double hover when cursor is on edge

Sorting of windows for render doesn't do 3 pass stupidly. Made code a little closer to what it should become with a layering system

Fix for using tooltips within popup #126  - will need to rewrite the sorting/layering system shortly.

Added popup demo code (commented out because I expect to make popup menu proper citizens by next version)

Maybe have side-effects on window contents? Unsure at this point.

Examples: OpenGL/GLFW: hide system cursor if imgui is soft-rendering cursor

Commented standard library include with the functions we use. Using ImFormatString in place of sprintf (part of #172)