inputs: add an 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 from a single message.

If the server doesn't support INPUTS_CAP_KEY_SCANCODE, the client is
responsible to handle a fallback mode with the exisiting KEY_DOWN and
KEY_UP messages.

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

spice/enums.h

@@ -491,6 +491,7 @@ enum {
     SPICE_MSGC_INPUTS_KEY_DOWN = 101,
     SPICE_MSGC_INPUTS_KEY_UP,
     SPICE_MSGC_INPUTS_KEY_MODIFIERS,
+    SPICE_MSGC_INPUTS_KEY_SCANCODE,
     SPICE_MSGC_INPUTS_MOUSE_MOTION = 111,
     SPICE_MSGC_INPUTS_MOUSE_POSITION,
     SPICE_MSGC_INPUTS_MOUSE_PRESS,

spice/protocol.h

@@ -130,6 +130,10 @@ enum {
     SPICE_DISPLAY_CAP_COMPOSITE,
 };
 
+enum {
+    SPICE_INPUTS_CAP_KEY_SCANCODE,
+};
+
 #include <spice/end-packed.h>
 
 #endif /* _H_SPICE_PROTOCOL */