inputs: add a INPUTS_KEY_SCANCODE message

Add a new arbitrary keyboard scancodes message.

For now, it will be used to avoid unwanted key repeatition when there
is jitter in the network and too much time between DOWN and UP
messages, instead the client will send the press & release scancode in
a sequence.

See also: https://bugzilla.redhat.com/show_bug.cgi?id=812347

spice.proto

@@ -934,6 +934,8 @@ channel InputsChannel : BaseChannel {
 	keyboard_modifier_flags modifiers;
     } @ctype(SpiceMsgcKeyModifiers) key_modifiers;
 
+    Data key_scancode;
+
     message {
 	int32 dx;
 	int32 dy;