WIP: Add simple print operation

[hzulla]

Hello,

this implements a very simple print function and closes #888.

There are still some issues with this, therefore I'm asking for comments.

The print settings are being reset each time the user creates a new or loads an existing document. For the first printing, the documents dimensions and resolution are compared to those of the printer's default document size.

If the document fits onto a single printer's page, it is scaled at the default 100%, even if that results in a small document printed on an mostly empty page.

If the document dimensions and resolution are too large, printing is scaled down accordingly to fit a single page.

If the user decides to use a different scaling factor or printer document size, the document will be tiled on as many printer pages as needed for the user's desired settings.

The print settings are then stored for the document and will be reused for the next call to the print function. If the user has made the document larger by then, the printing will be tiled accordingly.

Here are the issues where I request help/feedback.

• The accelerator Ctrl+P doesn't work as I would expect from reading the code, it will activate the TogglePreviewTool instead and I haven't found out why.
• My code first writes a temporary PNG file, uses that for printing and then deletes it later. Obviously, it should be possible to use the document directly for printing without a temporary image file, but my understanding of mypaint's cairo/gtk code is very limited and bit of help on how to make the document draw directly on the print page's cairo context would be welcome.
• The default DPI setting of a mypaint document is 72, which results in rather big prints. But that may be desirable for younger users, so it isn't much of an issue.
• Should the document always be printed on the center of the printer page?

[hzulla]

continuous-integration/appveyor/pr — AppVeyor build failed

Please advise. It appears to be unrelated to my patch?

[hzulla]

@achadwick do you have on opinion on this patch? I'd welcome your comments and would love to know if this is a desired functionality.