[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/backlight-control/backlight-control.c

@@ -7,7 +7,7 @@ static void event_cb(const void* value, void* ctx) {
     osThreadFlagsSet((osThreadId_t)ctx, BACKLIGHT_FLAG_ACTIVITY);
 }
 
-void backlight_control(void* p) {
+int32_t backlight_control(void* p) {
     // TODO open record
     const GpioPin* backlight_record = &backlight_gpio;
 
@@ -28,4 +28,6 @@ void backlight_control(void* p) {
             gpio_write(backlight_record, false);
         }
     }
+
+    return 0;
 }