SVG export should create one file per page, as PNG export does

SVG export of a multi-page score creates one file with all pages horizontally placed, side-by-side. This makes it difficult to print or display. For scores longer than two pages in length, the file is unusable (exceeds all standard paper widths, displays in tiny size on a screen.)

The SVG export should work exactly as the PNG export does: one file should be created for each page, with the page number postfixed to the score filename. This will allow the user to flow the pages as desired on the output device.

To recreate the issue:

1. Create a score with 3+ pages
2. In Page view, select File > Export > Files of Type: SVG
3. Print the resulting file, score will be unusably small

GIT commit: a65f06b