Fix clearing wxCB_READONLY wxComboBox in wxGTK.

wxComboBox::Clear() must call wxTextEntry::SetValue() explicitly instead of
calling its Clear() which just forwards back to wxComboBox own SetValue(),
which (correctly) doesn't work for read-only comboboxes when passed an empty
string.

Closes #16654.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78085 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -91,6 +91,7 @@ wxGTK:
 
 - Support building wxGTK3 under Windows (Kolya Kosenko).
 - Fix vertical cell alignment in wxDataViewCtrl.
+- Fix clearing of wxComboBox with wxCB_READONLY (Chuddah).
 
 wxMSW:
 

src/gtk/combobox.cpp

@@ -297,7 +297,12 @@ wxComboBox::GetClassDefaultAttributes(wxWindowVariant WXUNUSED(variant))
 
 void wxComboBox::Clear()
 {
-    wxTextEntry::Clear();
+    // Do not call wxTextEntry::Clear() here as it's implemented in terms of
+    // virtual SetValue() and so would call our own overridden version of this
+    // method, which wouldn't do the right thing in wxCB_READONLY case.
+    //
+    // Clear the text directly to avoid this.
+    wxTextEntry::SetValue(wxString());
     wxItemContainer::Clear();
 }