[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

lib/app-template/app-template.h

@@ -22,7 +22,7 @@ public:
     void release_state(void);
     bool get_event(TEvent* event, uint32_t timeout);
     void app_ready(void);
-    void exit(void);
+    uint8_t exit(void);
     void update_gui(void);
 };
 
@@ -34,7 +34,7 @@ template <class TState, class TEvent> AppTemplate<TState, TEvent>::AppTemplate()
     // TODO: use plain os mutex?
     if(!init_mutex(&state_mutex, &state, sizeof(TState))) {
         printf("cannot create mutex\n");
-        furiac_exit();
+        furi_check(0);
     }
 
     // open gui
@@ -100,10 +100,10 @@ template <class TState, class TEvent> void AppTemplate<TState, TEvent>::app_read
     gui_add_view_port(gui, view_port, GuiLayerFullscreen);
 }
 
-template <class TState, class TEvent> void AppTemplate<TState, TEvent>::exit(void) {
+template <class TState, class TEvent> uint8_t AppTemplate<TState, TEvent>::exit(void) {
     // TODO remove all view_ports create by app
     view_port_enabled_set(view_port, false);
-    osThreadExit();
+    return 255;
 }
 
 template <class TState, class TEvent> void AppTemplate<TState, TEvent>::update_gui(void) {