Add support for rendering SVG diagrams

This adds support for rendering SVG molecular diagrams in the
molecule details dialog. Now, if the molecule document in the
Mongo database contains an 'svg' field the data is loaded and
displayed instead of using the PNG diagram. This allows the
diagram to scale with the size of the dialog and looks better.

This adds a dependance on QtWebKit to perform the SVG rendering.
The QtSvg module was not used because it does not support scaled
rendering and has been deprecated.

Change-Id: I260d4c4b78e542ccf0c28ec0ce629b0743d00041
Add SvgGenerator class

This adds a new class which generates SVG diagrams for
molecules using openbabel.

Change-Id: I90c3da0a4ad91d34bd686c329f5a9f9da7a4a967
Merge topic 'svg-diagrams' into master

74912b15 Add SvgGenerator class
5c952940 Add support for rendering SVG diagrams