Proof-of-concept for auto-trimming of page margins

[robamler]

(Please note that I'm new to github. I'm not sure if a pull request is the correct way to notify about a half-baked new feature on a topic branch.)

This implementation follows the second approach outlined in #issuecomment-5596872 (see issue #20). Auto-trimming can be turned on and off with the boolean field PDFPageView.autoTrimOn, which, for now, is just a static constant.

Things that remain to be done:

• I'm not sure if this implementation is memory efficient. Maybe we could do with copying the image one less time.
• Coordinate transformation for forward/backward search and for annotations.
• Correct handling of the "fit page to screen" and "fit page width to screen width" zoom actions.
• Front end to turn auto-trimming on and off (toggle button).

[robamler]

Hi Boris, This implementation works well on my machine. Please let me know if you find any bugs or would like to have anything refactored. Robert

[Borisvl]

Hi, thank you very much for your effort. I will take a closer look at it in the next few days.