Add ability to load from SVG

[SeanDS]

Add ability to load existing SVG and interpret the components in it. This might require the version number to be embedded in files to change load behaviour for older files.

Hopefully loading a file and saving it without changes will produce an exact copy.

Could this be achieved via an XML namespace within the SVG file? This could provide hints to Optivis regarding connected nodes, etc.

The default save option should probably be 'Optivis SVG' but there should also be an option to select a 'Plain SVG' that doesn't contain extra namespace junk (i.e. the current system).