macos: enable running lan-mouse on macos (#42)

* macos: initial support

- adapted conditional compilation
- moved lan-mouse socket to ~/Library/Caches/lan-mouse-socket.sock instead of XDG_RUNTIME_DIR
- support for mouse input emulation
TODO: Keycode translation, input capture

src/backend/producer/macos.rs

@@ -0,0 +1,28 @@
+use crate::client::{ClientEvent, ClientHandle};
+use crate::event::Event;
+use crate::producer::EventProducer;
+use futures_core::Stream;
+use std::task::{Context, Poll};
+use std::{io, pin::Pin};
+
+pub struct MacOSProducer;
+
+impl MacOSProducer {
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+impl Stream for MacOSProducer {
+    type Item = io::Result<(ClientHandle, Event)>;
+
+    fn poll_next(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        Poll::Pending
+    }
+}
+
+impl EventProducer for MacOSProducer {
+    fn notify(&mut self, _event: ClientEvent) {}
+
+    fn release(&mut self) {}
+}