[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-test/gui-test.c

@@ -92,7 +92,7 @@ void text_input_callback(void* context, char* text) {
     next_view(context);
 }
 
-void gui_test(void* param) {
+int32_t gui_test(void* param) {
     (void)param;
     GuiTester* gui_tester = gui_test_alloc();
 
@@ -154,4 +154,6 @@ void gui_test(void* param) {
     while(1) {
         osDelay(1000);
     }
+
+    return 0;
 }