Extend AppBuilder api with add_system_set and similar methods (#1453)

Extend AppBuilder api with `add_system_set` and similar methods

Author: TheRawMeatball

Diff for: crates/bevy_app/src/app_builder.rs

@@ -6,7 +6,8 @@ use crate::{
 };
 use bevy_ecs::{
     clear_trackers_system, FromResources, IntoExclusiveSystem, IntoSystem, Resource, Resources,
-    RunOnce, Schedule, Stage, StageLabel, StateStage, SystemDescriptor, SystemStage, World,
+    RunOnce, Schedule, Stage, StageLabel, StateStage, SystemDescriptor, SystemSet, SystemStage,
+    World,
 };
 use bevy_utils::tracing::debug;
 
@@ -129,6 +130,10 @@ impl AppBuilder {
         self.add_system_to_stage(CoreStage::Update, system)
     }
 
+    pub fn add_system_set(&mut self, system_set: SystemSet) -> &mut Self {
+        self.add_system_set_to_stage(CoreStage::Update, system_set)
+    }
+
     pub fn add_system_to_stage(
         &mut self,
         stage_label: impl StageLabel,
@@ -138,6 +143,17 @@ impl AppBuilder {
         self
     }
 
+    pub fn add_system_set_to_stage(
+        &mut self,
+        stage_label: impl StageLabel,
+        system_set: SystemSet,
+    ) -> &mut Self {
+        self.app
+            .schedule
+            .add_system_set_to_stage(stage_label, system_set);
+        self
+    }
+
     pub fn add_startup_system(&mut self, system: impl Into<SystemDescriptor>) -> &mut Self {
         self.add_startup_system_to_stage(StartupStage::Startup, system)
     }

Diff for: crates/bevy_ecs/src/schedule/mod.rs

@@ -153,6 +153,16 @@ impl Schedule {
         self
     }
 
+    pub fn add_system_set_to_stage(
+        &mut self,
+        stage_label: impl StageLabel,
+        system_set: SystemSet,
+    ) -> &mut Self {
+        self.stage(stage_label, |stage: &mut SystemStage| {
+            stage.add_system_set(system_set)
+        })
+    }
+
     pub fn stage<T: Stage, F: FnOnce(&mut T) -> &mut T>(
         &mut self,
         label: impl StageLabel,