Fix TSaveSourceMgr to preview HTML5 files correctly

HTML 5 files are actually previewed as XHTML because support for HTML5
might not be properly implemented by the IE based MS web browser
control.

TSaveSourceMgr was also changed to explicitly render pascal file as
plain text for previewing. Although this change is not strictly
necessary since pascal and plain text file types render the same, it
was done for consitency and clarity.

Author: delphidabbler

Src/USaveSourceMgr.pas

@@ -331,16 +331,23 @@ procedure TSaveSourceMgr.PreviewHandler(Sender: TObject);
     dtHTML,       // sfXHTML
     dtRTF         // sfRTF
   );
+  PreviewFileTypeMap: array[TPreviewDocType] of TSourceFileType = (
+    sfText,       // dtPlainText
+    sfXHTML,      // dtHTML
+    sfRTF         // dtRTF
+  );
 var
-  FileType: TSourceFileType;  // type of source file to preview
+  PreviewFileType: TSourceFileType;  // type of source file to preview
+  PreviewDocType: TPreviewDocType;   // type of file to be generated for preview
 begin
-  FileType := FileTypeFromFilterIdx;
+  PreviewDocType := PreviewDocTypeMap[FileTypeFromFilterIdx];
+  PreviewFileType := PreviewFileTypeMap[PreviewDocType];
   // Display preview dialog box. We use save dialog as owner to ensure preview
   // dialog box is aligned over save dialog box
   TPreviewDlg.Execute(
     fSaveDlg,
-    GenerateOutput(FileType),
-    PreviewDocTypeMap[FileType],
+    GenerateOutput(PreviewFileType),
+    PreviewDocType,
     GetDocTitle
   );
 end;