Read page size from svg xml element

[sporteries]

This MR addresses opening svg file with widths and heights expressed in mm instead of px.

Modifications:

• function loadFile call readPageSize which is reponsible to update mediaSize with just loaded document
• readPageSize open the file with QDomDocument and extracts fields width and height from svg tag
• stringSizeToMm convert a size string to a mm float value testing if its unit is mm or pixel (in this case dividing by ppm)

[Timmmm]

Ah I think I actually already did this in develop. Sorry I really need to merge that into master but there's an issue that I haven't debugged yet where it leaves the cutter in some kind of broken state after one cut and you need to turn it off and on again. :-/