[Base] Moved IsUseNexusForGameBarEnabled from emulator_window to system.h

This will remove direct usage of system specific code from emulator_window

src/xenia/base/system_win.cc

@@ -64,4 +64,17 @@ bool SetProcessPriorityClass(const uint32_t priority_class) {
                           xeniaToWindowsPriorityClassMapping[priority_class]);
 }
 
+bool IsUseNexusForGameBarEnabled() {
+  const LPCWSTR reg_path = L"SOFTWARE\\Microsoft\\GameBar";
+  const LPCWSTR key = L"UseNexusForGameBarEnabled";
+
+  DWORD value = 0;
+  DWORD dataSize = sizeof(value);
+
+  RegGetValue(HKEY_CURRENT_USER, reg_path, key, RRF_RT_DWORD, nullptr, &value,
+              &dataSize);
+
+  return static_cast<bool>(value);
+}
+
 }  // namespace xe