Make BrowserDialog code look more like EffectUIHost code

Specifically in the area of closing a window.  Added mDismissed member which can stop double dismissal.  Perhaps more relevant (?) still do an EndModal IF IsModal() is true.  These changes are in response to feedback from Paul that simply removing EndModal (which seems good on Windows) causes problems (in TimerRecord!) on Mac.

src/widgets/LinkingHtmlWindow.cpp

@@ -56,7 +56,11 @@ void BrowserDialog::OnBackward(wxCommandEvent & WXUNUSED(event))
 
 void BrowserDialog::OnClose(wxCommandEvent & WXUNUSED(event))
 {
-   //EndModal(wxID_CANCEL);
+   if (IsModal() && !mDismissed)
+   {
+      mDismissed = true;
+      EndModal(wxID_CANCEL);
+   }
    auto parent = GetParent();
 
 #ifdef __WXMAC__

src/widgets/LinkingHtmlWindow.h

@@ -52,6 +52,7 @@ public:
 
 
    HtmlWindow * mpHtml;
+   bool mDismissed{};
    DECLARE_EVENT_TABLE()
 };