dashboard: add option to disable panels

In /etc/mmc/plugins/dashboard.ini use the disabled_panels option to
disable specific panels, eg:

  disabled_panels = shortcuts services

The python part of panels is now *mandatory*. The minimal code is:

  from mmc.plugins.dashboard.panel import Panel

    class InventoryPanel(Panel):
      pass

Then every panel should be registered in the DashboardManager like this:

  try:
    from mmc.plugins.dashboard.manager import DashboardManager
    from mmc.plugins.inventory.panel import InventoryPanel
    DM = DashboardManager()
    DM.register_panel(InventoryPanel("inventory"))
  except ImportError:
    pass

Refs #1969