Fix #557: Add Base64 export

[Photon89]

This adds the Base64 export. Similar to the PDF or PS export. There is a corresponding entry in the "File" menu to use it.

Base64 stuff taken from https://stackoverflow.com/questions/11301854/perl-convert-image-to-base64

[DarthGandalf]

Base64 is not an image format. Base64 encodes a stream of bytes to stream of text. These bytes could be whatever: PNG, JPG, PDF, SVG even.

What exactly does this export mean?

[Photon89]

Well, it takes the file in whatever format (not a pixbuf) and makes a base64 encoded text stream out of it. To be honest, I don't know, why this is useful, but somebody requested it (see linked issue).

[DarthGandalf]

I don't know, why this is useful

I'd be against feature creep, especially if even you don't know what's the purpose of such feature

[Photon89]

I propose to wait for a reply from the issue reporter and then see if it is a valid use case.

[DarthGandalf]

Sure

[LespiletteMaxime]

I gave a use case here #557 (comment)