Handle old panel cmdline/dbus arguments in System

The System panel subpages used to be CcPanels. They still have their own desktop file so that they can be findable in Search.

A future goal is to decouple our Search model from our panel list model, so that we can better handle panel subpages and other more grained search results.

For now in these changes I overwrite the start_id and parameters for activation of subpages of the "System" panel. This way, legacy cmdline arguments such as gnome-control-center datetime get remaped to gnome-control-center system datetime. This also works for dbus activation.

These cmdline arguments are now deprecated and applications should have a time-window to port away from it.

Fixes #2930 (closed)