Add option to select canvas portrait/landscape orientation to Resize Canvas dialog.

Description of the feature

As proposed by Jehan in !406 (merged).

If I could propose something though to improve (#1140 (closed)) even more, it would be nice to have the 2 little "landscape" and "portrait" icon-buttons as we have in the "Create a New Image" dialog.

Use cases

Typically if I want to work, say in A4, it's not always in portrait. Depends on the project.

Notes

Adding 4. Newcomers as it shouldn't be too hard to implement.