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/backend/producer.rs

@@ -0,0 +1,6 @@
+#[cfg(feature = "wayland")]
+pub mod wayland;
+#[cfg(windows)]
+pub mod windows;
+#[cfg(feature = "x11")]
+pub mod x11;