Implement From<u64> for WindowId and vise-versa

This should help downstream applications to expose WindowId to the end
users via e.g. IPC to control particular windows in multi window
systems.

src/platform_impl/linux/mod.rs

@@ -162,19 +162,23 @@ pub enum Window {
 }
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub enum WindowId {
-    #[cfg(feature = "x11")]
-    X(x11::WindowId),
-    #[cfg(feature = "wayland")]
-    Wayland(wayland::WindowId),
+pub struct WindowId(u64);
+
+impl From<WindowId> for u64 {
+    fn from(window_id: WindowId) -> Self {
+        window_id.0
+    }
+}
+
+impl From<u64> for WindowId {
+    fn from(raw_id: u64) -> Self {
+        Self(raw_id)
+    }
 }
 
 impl WindowId {
     pub const unsafe fn dummy() -> Self {
-        #[cfg(feature = "wayland")]
-        return WindowId::Wayland(wayland::WindowId::dummy());
-        #[cfg(all(not(feature = "wayland"), feature = "x11"))]
-        return WindowId::X(x11::WindowId::dummy());
+        Self(0)
     }
 }
 
@@ -314,7 +318,12 @@ impl Window {
 
     #[inline]
     pub fn id(&self) -> WindowId {
-        x11_or_wayland!(match self; Window(w) => w.id(); as WindowId)
+        match self {
+            #[cfg(feature = "wayland")]
+            Self::Wayland(window) => window.id(),
+            #[cfg(feature = "x11")]
+            Self::X(window) => window.id(),
+        }
     }
 
     #[inline]