Update smithay-client-toolkit to 'v0.15.0'

This commit also drops 'Theme' trait with its support types
in favor of 'FallbackFrame' meaning that winit will use some
predefined frame for the time being, since porting 'ConceptFrame'
will require adding font rendering librarires right into winit,
which is not desired.

Fixes #1889.

src/platform_impl/linux/wayland/output.rs

@@ -112,7 +112,7 @@ impl Eq for MonitorHandle {}
 
 impl PartialOrd for MonitorHandle {
     fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
-        Some(self.cmp(&other))
+        Some(self.cmp(other))
     }
 }