Add Support For Printing

[jimorc]

Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Fyne provides drawing capabilities, including text. What is not supported is the ability to print the resulting drawing.

I do realize that printing is more typical of a framework than a toolkit, but it would allow a much larger range of applications to be built using fyne.

Is it possible to construct a solution with the existing API?

Well, yes, but this involves saving the drawing as some sort of image and then opening a separate app to print the image, so not really.

Describe the solution you'd like to see.

Add printing capabilities to fyne. Most likely this would require printing of or from a canvas.