[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/gui/gui.c

@@ -280,7 +280,7 @@ Gui* gui_alloc() {
     return gui;
 }
 
-void gui_task(void* p) {
+int32_t gui_task(void* p) {
     Gui* gui = gui_alloc();
 
     furi_record_create("gui", gui);
@@ -302,4 +302,6 @@ void gui_task(void* p) {
             gui_redraw(gui);
         }
     }
+
+    return 0;
 }