[simu] Refactor simu startup and shutdown handling (for 2.3 branch):

* Use pwrCheck() as primary control point for shutdown, centralizes thread control in simpgmspace module;
 * Remove main_thread_running in favor of separate variables for startup mode and shutdown flag;
 * Don't start a thread for ARM startup, just run simuMain() directly (the thread just exits anyway);
 * Refactor SIMU_SLEEP() macro, and remove all unnecessary uses of it, also remove SIMU_SLEEP_NORET;
 * Add startType param to opentxStart() to control splash and startup checks;
 * Fixes backlight never turning off in simulator with "SWITCH" power button type (pwrPressed() was always true);
 * Fixes simu shutdown when model checklist is displayed at startup;
 * Adds stubs to possibly simulate a "soft" power button in the future.

radio/src/gui/common/colorlcd/widgets.cpp

@@ -82,6 +82,7 @@ void runFatalErrorScreen(const char * message)
       uint32_t pwr_check = pwrCheck();
       if (pwr_check == e_power_off) {
         boardOff();
+        return;  // only happens in SIMU, required for proper shutdown
       }
       else if (pwr_check == e_power_press) {
         refresh = true;
@@ -89,7 +90,6 @@ void runFatalErrorScreen(const char * message)
       else if (pwr_check == e_power_on && refresh) {
         break;
       }
-      SIMU_SLEEP_NORET(1);
     }
   }
 }