SpiceAgent: Add a new spice agent service :^)

A SPICE agent communicates with the host OS to provide nifty features
like clipboard sharing :^)

This patch implements only plain-text clipboard sharing.

See: github.com/freedesktop/spice-protocol/blob/master/spice/vd_agent.h

Userland/Services/SpiceAgent/CMakeLists.txt

@@ -0,0 +1,13 @@
+serenity_component(
+    SpiceAgent
+    TARGETS SpiceAgent
+)
+
+set(SOURCES
+    main.cpp
+    SpiceAgent.cpp
+    ClipboardServerConnection.cpp
+)
+
+serenity_bin(SpiceAgent)
+target_link_libraries(SpiceAgent LibGfx LibCore LibIPC)