[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/power/power.c

@@ -177,7 +177,7 @@ void power_cli_otg_off(string_t args, void* context) {
     api_hal_power_disable_otg();
 }
 
-void power_task(void* p) {
+int32_t power_task(void* p) {
     (void)p;
     Power* power = power_alloc();
 
@@ -223,4 +223,6 @@ void power_task(void* p) {
         view_port_enabled_set(power->usb_view_port, api_hal_power_is_charging());
         osDelay(1024);
     }
+
+    return 0;
 }