Add multiple screens with page up down

[hranmuthu]

Initially start with page 0. You can pan and draw anything on a page, when page down is pressed, save the current drawing (in memory or on a file) and clear current drawing and create page 1. You can draw again. if you press page down again, save the current page too and create page 2 and so on. You can use page up or down to browse through the pages or create a new page with page down when you reach the last page.

This will give us very powerful capability to maintain multiple drawings in one session.

[pubpub-zz]

@hranmuthu
I've added this feature in my fork. A beta version is available in this thread pubpub-zz#258