[panel] Reset Panel if dbus connection to Dash is lost (e.g. Shell process gets killed):
- connect to serviceUnregistered of QDBusServiceWatcher in both DashClient and HudClient
- call updateWidgets in AppNameApplet applet and updatePallette in PanelPaletteManager
on loss of dbus connection
- DashClient and HudClient reconnect to dbus after dbus service becomes available. Fixes: https://bugs.launchpad.net/bugs/953168. Approved by .