Replace input filter with event filter

This commit applies the transition from the "Input" session to the "Event"
session to the event-filtering mechansim. The functionality of the
input_filter is now provided by the event_filter. The event filter
requests only one "Event" session as destination for the filter result,
which is usually routed to the nitpicker GUI server. It provides an
"Event" service to which any number of event sources can connect.

The configuration of the filter chain remains almost the same. Only the
declaration of the <input> nodes is no longer needed. Instead, the
configuration must specify <policy> nodes, which define the mapping of
"Event" clients (event sources) to the inputs used in the filter chain.

The patch adjusts all uses of the nitpicker GUI server accordingly such
that the event filter reports events to nitpicker's event service
instead of having nitpicker request an "Input" session. This dissolves
the dependency of nitpicker from input drivers.

Issue #3827

tool/autopilot.list

@@ -14,7 +14,7 @@ fs_query
 gdb_monitor
 ieee754
 init_smp
-input_filter
+event_filter
 libc_vfs_fs_ext2
 log_core
 lwip