Document run_event_loop() and quit_event_loop()

The C++ versions were there before, but undocumented. The Rust version
for quit() was missing.

CHANGELOG.md

@@ -21,6 +21,7 @@ All notable changes to this project will be documented in this file.
 
  - Warnings are now shown in the online code editor.
  - `sixtyfps::invoke_from_event_loop` was added to the C++ and Rust APIs, to run a function in the UI thread from any thread.
+ - `sixtyfps::run_event_loop()` and `sixtyfps::quit_event_loop()` were added to the Rust and C++ APIs to start and quit the main event loop.
  - `z` property on items.
  - The type in two-way bindings can now be omitted.
  - It's possible to declare aliases for callbacks (`callback clicked <=> other.clicked;`)

api/sixtyfps-cpp/include/sixtyfps.h

@@ -39,8 +39,8 @@ using cbindgen_private::ItemVTable;
 using ComponentRef = vtable::VRef<private_api::ComponentVTable>;
 using ItemRef = vtable::VRef<private_api::ItemVTable>;
 using ItemVisitorRefMut = vtable::VRefMut<cbindgen_private::ItemVisitorVTable>;
-using cbindgen_private::ItemWeak;
 using cbindgen_private::ComponentRc;
+using cbindgen_private::ItemWeak;
 using cbindgen_private::TraversalOrder;
 }
 
@@ -636,11 +636,18 @@ struct VersionCheckHelper
 };
 }
 
+/// Enters the main event loop. This is necessary in order to receive
+/// events from the windowing system in order to render to the screen
+/// and react to user input.
 inline void run_event_loop()
 {
     cbindgen_private::sixtyfps_run_event_loop();
 }
 
+/// Schedules the main event loop for termination. This function is meant
+/// to be called from callbacks triggered by the UI. After calling the function,
+/// it will return immediately and once control is passed back to the event loop,
+/// the initial call to sixtyfps::run_event_loop() will return.
 inline void quit_event_loop()
 {
     cbindgen_private::sixtyfps_quit_event_loop();

api/sixtyfps-rs/lib.rs

@@ -263,6 +263,14 @@ pub fn run_event_loop() {
         .run_event_loop(sixtyfps_corelib::backend::EventLoopQuitBehavior::QuitOnLastWindowClosed);
 }
 
+/// Schedules the main event loop for termination. This function is meant
+/// to be called from callbacks triggered by the UI. After calling the function,
+/// it will return immediately and once control is passed back to the event loop,
+/// the initial call to [`run_event_loop()`] will return.
+pub fn quit_event_loop() {
+    sixtyfps_rendering_backend_default::backend().quit_event_loop();
+}
+
 /// Adds the specified function to an internal queue, notifies the event loop to wake up.
 /// Once woken up, any queued up functors will be invoked.
 ///