Add option to crop white borders

[MatejSpindler]

Some printers in CAD programs can only print to standard page sizes, if you want to create a PDF of custom size like extended A3, usual workflow is to print to A2 or A1 and then crop extra white border.

With this change this operation is completed with just one click.

Size of white borders is determined by rendering the page with Cairo and then examining pixels for color other then white. This seems like a brute force approach, but I couldn't find any other solution.

[jeromerobert]

Thank you.

• The pdfarranger.pdfarranger module is already too bloated. May be I should move crop_white_borders and crop_page_dialog to a pdfarranger.crop module. I'll do somethink like that, then merge and adapt your PR.

• Would you mind if I move this action to the crop dialog ? I imagine an autocrop button on the crop dialog which automatically set the crop parameters. This would avoid to add a new action in the menus and this would allow the user to tweak the result of the autocrop. The downside is that it add extra clicks (open/close dialog) when you just want autocrop. This is not for now as the crop feature need a rework: space units instead of %, allow extends (negative crop).

[MatejSpindler]

Sorry for the late answer. It was a busy week.

Regarding your first point, sure that module is getting big and hard to navigate. Splitting it is a good idea,

For the second point, it seems to me crop dialogue for a selection of pages applies the same crop to all pages. How do you plan to implement different per page crop regions in the selection?

The attached test file is a good representation of my daily workflow:
AutoCrop-test.pdf

[jeromerobert]

Merged: 3320f4f c5c41cb

[mara004]

Wow, that's a nice feature.