Add `close_on_esc()` system to all examples (#58)

Make it easy to exit an example with the ESC key by adding the built-in
`bevy::window::close_on_esc()` system to all examples.

examples/colormaterial_color.rs

@@ -15,6 +15,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_startup_system(setup)
         .run();

examples/menu.rs

@@ -13,6 +13,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_plugin(WorldInspectorPlugin::new())
         .add_startup_system(setup)

examples/sequence.rs

@@ -14,6 +14,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_startup_system(setup)
         .add_system(update_text)

examples/sprite_color.rs

@@ -11,6 +11,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_startup_system(setup)
         .run();

examples/text_color.rs

@@ -14,6 +14,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_startup_system(setup)
         .run();

examples/transform_rotation.rs

@@ -13,6 +13,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_plugin(InspectorPlugin::<Options>::new())
         .add_startup_system(setup)

examples/transform_translation.rs

@@ -13,6 +13,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_plugin(InspectorPlugin::<Options>::new())
         .add_startup_system(setup)

examples/ui_position.rs

@@ -13,6 +13,7 @@ fn main() {
             ..default()
         })
         .add_plugins(DefaultPlugins)
+        .add_system(bevy::window::close_on_esc)
         .add_plugin(TweeningPlugin)
         .add_plugin(InspectorPlugin::<Options>::new())
         .add_startup_system(setup)