Don't disable main application window in wxProgressDialog

Since 58f90d36a0, the main application
window was disabled even if no parent was specified when creating
wxProgressDialog, which was a change in behaviour compared to 3.1.0 and
earlier versions.

Revert this change and don't disable any windows if neither parent nor
wxPD_APP_MODAL is given.

Closes #18189.

interface/wx/progdlg.h

@@ -105,7 +105,11 @@ public:
             In the generic implementation the progress bar is constructed
             only if this value is greater than zero.
         @param parent
-            Parent window.
+            Parent window. It will be disabled while this dialog is shown if
+            non-null (whether @c wxPD_APP_MODAL is specified or not). Note that
+            if you specify null parent and don't use @c wxPD_APP_MODAL, you
+            need to take care to avoid reentrancies, i.e. avoiding showing the
+            progress dialog again while this one is shown.
         @param style
             The dialog style. See wxProgressDialog.
     */