[FL-3846] Event Loop Timers (#3721)

* Implement POC event loop tmers (not all edge cases are handled)
* Use a separate ready list to allow for (re)starting and stopping of timers from callback
* Improve the test application
* Improve timer API and test application
* Improve timeout calculation logic
* Improve timer API, update documentation
* Fix API usage error
* Update doxygen comments
* Revert the old (correct) check
* Improve function naming
* Check whether a timer was on the expired list before processing it
* Implement tick callback
* Add critical sections to improve timer consistency
* Simplify event loop timer API
* Remove redundant search
* Refactor timer logic, use message queue
* Simplify FuriEventLoopTimer API
* Improve event loop timer logic
* Update the f18 target
* Remove superfluous clears
* Correct f18 api symbols
* Fix doxygen comments
* Update .pvsconfig
* Use a double push list instead of deque
* Update .pvsconfig
* Add pending callback functionality
* Restore unprocessed flags when applicable
* Refactor Dolphin app to use FuriEventLoop
* Improve naming
* Update naming some more
* Fix a typo Co-authored-by: Silent <CookiePLMonster@users.noreply.github.com>
* Fix wait time in example
* Bump API version
* Debug: multiple of 25 timings in event loop blink test
* Separate FuriEventLoopTimer to its own set of files
* Improve start time calculation for periodic timers
* Do not use dynamic allocations for timer requests
* Split the tick functionality in separate files, rearrange code
* Improve timer queue handling
* Properly reset GPIO pins in the test app
* Properly initialise GPIO pins in the test app too
* Furi: variable naming in event loop
* Furi: fix spelling in event loop

Co-authored-by: あく <alleteam@gmail.com>
Co-authored-by: Silent <CookiePLMonster@users.noreply.github.com>

furi/core/event_loop.h

@@ -34,7 +34,7 @@ typedef struct FuriEventLoop FuriEventLoop;
  * Couple things to keep in mind:
  * - You can have 1 event_loop per 1 thread
  * - You can not use event_loop instance in the other thread
- * - Do not use blocking api to query object delegated to Event Loop
+ * - Do not use blocking API to query object delegated to Event Loop
  *
  * @return     The Event Loop instance
  */
@@ -72,8 +72,10 @@ typedef void (*FuriEventLoopTickCallback)(void* context);
 
 /** Set Event Loop tick callback
  *
- * Tick callback called after specified inactivity time. It's not periodic. If
- * Event Loop is busy then ticks will be skipped.
+ * Tick callback is called periodically after specified inactivity time.
+ * It acts like a low-priority timer: it will only fire if there is time
+ * left after processing the synchronization primitives and the regular timers.
+ * Therefore, it is not monotonic: ticks will be skipped if the event loop is busy.
  *
  * @param      instance  The Event Loop instance
  * @param[in]  interval  The tick interval
@@ -86,6 +88,32 @@ void furi_event_loop_tick_set(
     FuriEventLoopTickCallback callback,
     void* context);
 
+/*
+ * Deferred function call API
+ */
+
+/**
+ * @brief Timer callback type for functions to be called in a deferred manner.
+ *
+ * @param[in,out] context pointer to a user-specific object that was provided during
+ *                        furi_event_loop_pend_callback() call
+ */
+typedef void (*FuriEventLoopPendingCallback)(void* context);
+
+/**
+ * @brief Call a function when all preceding timer commands are processed
+ *
+ * This function may be useful to call another function when the event loop has been started.
+ *
+ * @param[in,out] instance pointer to the current FuriEventLoop instance
+ * @param[in] callback pointer to the callback to be executed when previous commands have been processed
+ * @param[in,out] context pointer to a user-specific object (will be passed to the callback)
+ */
+void furi_event_loop_pend_callback(
+    FuriEventLoop* instance,
+    FuriEventLoopPendingCallback callback,
+    void* context);
+
 /*
  * Message queue related APIs
  */