LibIPC: Add support for transferring doubles over IPC messages

I'm still wondering why nobody did this yet :^) Also changes the use of
unions for the more cleaner / less undefined AK::bit_cast.

Userland/Libraries/LibIPC/Decoder.h

@@ -41,6 +41,7 @@ public:
     bool decode(i32&);
     bool decode(i64&);
     bool decode(float&);
+    bool decode(double&);
     bool decode(String&);
     bool decode(ByteBuffer&);
     bool decode(URL&);