Fix handling keys also used as accelerators in wxMSW wxComboBox

wxComboBox::MSWShouldPreProcessMessage() didn't take into account many
keys that must be handled in wxComboBox even if they're used as
accelerators, including plain (i.e. without Ctrl modifier) Delete, Home
and End that are used by the embedded text control, if there is one.

Fix this by reusing wxTextCtrl::MSWShouldPreProcessMessage() which
already handled these keys correctly, by moving it to wxTextEntry, which
can be used from both classes.

Also add a check for wxCB_READONLY to prevent overriding the
accelerators using the keys that the combobox doesn't need when there is
no text control in it.

Closes #19227.

src/msw/textentry.cpp

@@ -1072,4 +1072,74 @@ bool wxTextEntry::ClickDefaultButtonIfPossible()
                     wxWindow::MSWGetDefaultButtonFor(GetEditableWindow()));
 }
 
+bool wxTextEntry::MSWShouldPreProcessMessage(WXMSG* msg) const
+{
+    // check for our special keys here: if we don't do it and the parent frame
+    // uses them as accelerators, they wouldn't work at all, so we disable
+    // usual preprocessing for them
+    if ( msg->message == WM_KEYDOWN )
+    {
+        const WPARAM vkey = msg->wParam;
+        if ( HIWORD(msg->lParam) & KF_ALTDOWN )
+        {
+            // Alt-Backspace is accelerator for "Undo"
+            if ( vkey == VK_BACK )
+                return false;
+        }
+        else // no Alt
+        {
+            // we want to process some Ctrl-foo and Shift-bar but no key
+            // combinations without either Ctrl or Shift nor with both of them
+            // pressed
+            const int ctrl = wxIsCtrlDown(),
+                      shift = wxIsShiftDown();
+            switch ( ctrl + shift )
+            {
+                default:
+                    wxFAIL_MSG( wxT("how many modifiers have we got?") );
+                    wxFALLTHROUGH;
+
+                case 0:
+                    switch ( vkey )
+                    {
+                        case VK_DELETE:
+                        case VK_HOME:
+                        case VK_END:
+                            return false;
+                    }
+                    break;
+
+                case 1:
+                    // either Ctrl or Shift pressed
+                    if ( ctrl )
+                    {
+                        switch ( vkey )
+                        {
+                            case 'A':
+                            case 'C':
+                            case 'V':
+                            case 'X':
+                            case VK_INSERT:
+                            case VK_DELETE:
+                            case VK_HOME:
+                            case VK_END:
+                                return false;
+                        }
+                    }
+                    else // Shift is pressed
+                    {
+                        if ( vkey == VK_INSERT || vkey == VK_DELETE )
+                            return false;
+                    }
+                    break;
+
+                case 2:
+                    break;
+            }
+        }
+    }
+
+    return true;
+}
+
 #endif // wxUSE_TEXTCTRL || wxUSE_COMBOBOX