Set opt-level=3 for dependencies in debug builds

Cargo.toml

@@ -5,3 +5,12 @@ members = [
   "srv",
   "gui",
 ]
+
+# Enable max optimizations for dependencies, but not for our code:
+# Very helpful when developing in Slint.
+[profile.dev.package."*"]
+opt-level = 3
+
+# Enable only a small amount of optimization in debug mode
+[profile.dev]
+opt-level = 1