panorama: add some help text to general settings tab
panorama: make backends selectable for each dashboard
panorama: use global backend selector
panorama: reworked backend selection

dashboards now globally select a subset of backends, the site panel
then switches display only for the current dashboard. Panlets can
filter backends even further.
panorama: make filter use the current active backends
panorama: code cleanup
panorama: code cleanup
panorama: render settings panel only on first click
panorama: code cleanup
fixed display issue in latest firefox (updated dagre library)