signal: show Windows popup window on crash (#410)

## Link to GitHub Issue, if one exists
#345 

## Description of change
Show a MessageBox when signal detects a crash.

## Testing
Tested full screen and windowed games.

src/spice2x/util/deferlog.cpp

@@ -72,4 +72,25 @@ namespace deferredlogs {
             log_warning("troubleshooter", "{}", msg);
         });
     }
+
+    void show_popup_for_crash() {
+        static std::once_flag shown;
+        std::call_once(shown, []() {
+            const std::string title = "spice2x (" + to_string(VERSION_STRING_CFG) + ") - crash handler";
+            std::string text;
+            text += "Game has crashed.\n\n";
+            text += "Check log.txt and look for error messages near the end of file.\n\n";
+            text += "Unsure what to do next?\n";
+            text += "  * update spice2x to the latest version\n";
+            text += "  * check the FAQ on spice2x wiki on github\n";
+            text += "  * do NOT screenshot this, instead, share log.txt with someone and ask for help\n\n";
+            text += "Press Enter, Esc, Alt+F4, or click OK to exit. Otherwise, game will close in 30 seconds.";
+
+            MessageBox(
+                nullptr,
+                text.c_str(),
+                title.c_str(),
+                MB_OK | MB_ICONERROR | MB_TASKMODAL | MB_SETFOREGROUND | MB_TOPMOST);
+        });
+    }
 }