Improve SVG integration

[Ingvord]

• svg does not initializes when not visible i.e. must be always visible main tab
• process attributes/commands/pipes
• as per 0.7 svg is reloaded (or at least svg.onload) every time the widget is shown
• preserve pan and zoom (see above)
• dynamically set ids, i.e. svg has aliases, where alias is mapped to id when clicked
• provide attribute/command/pipe feedback e.g. state/status
• show/hide layers while zooming