[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/bt/bt.c

@@ -37,7 +37,7 @@ void bt_cli_info(string_t args, void* context) {
     string_clear(buffer);
 }
 
-void bt_task() {
+int32_t bt_task() {
     Bt* bt = bt_alloc();
 
     furi_record_create("bt", bt);
@@ -48,4 +48,6 @@ void bt_task() {
         view_port_enabled_set(bt->statusbar_view_port, api_hal_bt_is_alive());
         osDelay(1024);
     }
+
+    return 0;
 }