[FL-872] Furi, API-HAL, App-Loader cleanup and improvements (#334)

* Furi: replace obsolete furiac_exit with osThreadExit, drop obsolete apis and test. Rename systemd to flipper and move to separate file, cleanup. ApiHal: Rename timebase to os and move freertos hooks there, move insomnia api to power module.
* Furi: new thread helper
* Furi: cleanup thread documentation
* Flipper, AppLoader: update to use FuriThread. Update tasks signatures to match FuriThreadCallback signature.
* F4: rename API_HAL_TIMEBASE_DEBUG to API_HAL_OS_DEBUG
* Applications: rename FuriApplication to FlipperApplication, use FuriThreadCallback signature for apps.
* C++ app template sample, new exit method

applications/dolphin/dolphin.c

@@ -129,7 +129,7 @@ void dolphin_deed(Dolphin* dolphin, DolphinDeed deed) {
     furi_check(osMessageQueuePut(dolphin->event_queue, &event, 0, osWaitForever) == osOK);
 }
 
-void dolphin_task() {
+int32_t dolphin_task() {
     Dolphin* dolphin = dolphin_alloc();
 
     Gui* gui = furi_record_open("gui");
@@ -163,4 +163,6 @@ void dolphin_task() {
             dolphin_state_save(dolphin->state);
         }
     }
+
+    return 0;
 }