enable conditional compilation for all backends

To reduce binary size one can now enable only specific backends, e.g.
wayland or x11 via cargo features

Additionally adds stubs for libei and xdg-desktop-portal backends

src/producer.rs

@@ -0,0 +1,52 @@
+#[cfg(unix)]
+use std::env;
+use std::{thread::{JoinHandle, self}, sync::mpsc::SyncSender};
+
+use crate::{client::{Client, ClientHandle}, event::Event, request::Server};
+
+use crate::backend::producer;
+
+enum Backend {
+    Wayland,
+    X11,
+}
+
+pub fn start(
+    produce_tx: SyncSender<(Event, ClientHandle)>,
+    clients: Vec<Client>,
+    request_server: Server,
+) -> JoinHandle<()> {
+    thread::Builder::new()
+        .name("event producer".into())
+        .spawn(move || {
+            #[cfg(windows)]
+            producer::windows::run(produce_tx, request_server, clients);
+
+            #[cfg(unix)]
+            let backend = match env::var("XDG_SESSION_TYPE") {
+                Ok(session_type) => match session_type.as_str() {
+                    "x11" => Backend::X11,
+                    "wayland" => Backend::Wayland,
+                    _ => panic!("unknown XDG_SESSION_TYPE"),
+                },
+                Err(_) => panic!("could not detect session type: XDG_SESSION_TYPE environment variable not set!"),
+            };
+
+            #[cfg(unix)]
+            match backend {
+                Backend::X11 => {
+                    #[cfg(not(feature = "x11"))]
+                    panic!("feature x11 not enabled");
+                    #[cfg(feature = "x11")]
+                    producer::x11::run(produce_tx, request_server, clients);
+                }
+                Backend::Wayland => {
+                    #[cfg(not(feature = "wayland"))]
+                    panic!("feature wayland not enabled");
+                    #[cfg(feature = "wayland")]
+                    producer::wayland::run(produce_tx, request_server, clients);
+                }
+            }
+        })
+        .unwrap()
+}