[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/tests/test_index.c

@@ -5,7 +5,7 @@
 
 int run_minunit();
 
-void flipper_test_app(void* p) {
+int32_t flipper_test_app(void* p) {
     // create pins
     GpioPin red = {.pin = LED_RED_Pin, .port = LED_RED_GPIO_Port};
     GpioPin green = {.pin = LED_GREEN_Pin, .port = LED_GREEN_GPIO_Port};
@@ -38,7 +38,5 @@ void flipper_test_app(void* p) {
         gpio_write(blue_record, true);
     }
 
-    set_exitcode(exitcode);
-
-    furiac_exit(NULL);
+    return 0;
 }