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.h

@@ -11,4 +11,5 @@ namespace deferredlogs {
 
     void defer_error_messages(std::initializer_list<std::string> messages);
     void dump_to_logger(bool is_crash=false);
+    void show_popup_for_crash();
 }