Set sans-serif as default font for text elements

Set sans-serif as a fallback font for text elements. sans-serif is part
of the SVG spec and should be included in any SVG renderer. This might
also fix the [CANT] error in Illustrator.
Merge branch 'master' into hotfix
Merge branch 'hotfix'
SVG file chooser expects *.svg file name

Added an extension filter to the file chooser for SVGs.
Merge branch 'hotfix'
Merge branch 'master' of https://github.com/CIRDLES/topsoil