DPIswitcher: incorrect results with percentage-sized objects in some cases

Testing with DPIswitcher (following work on #175 (closed), !204 (closed)), it appears to incorrect results in a variety of cases including but not limited to:

1. Objects in layers with size/position specified as percent
2. Use objects referencing percentage-sized objects

(1) is a result of only checking top-level objects in the SVG. I suspect that the DPIswitcher may need to traverse the tree recursively to identify deeply nested objects that may have size specified with percentages of the document.

(2) is a result of not inspecting the dimensions of referenced objects.

Test example file:

After running through DPI 96 to 90, the resulting file is:

This displays several objects in the wrong places and/or at the wrong sizes.

I will work on this issue as part of continuing work on #175 (closed), !204 (closed).