Prevent editing value on focus when `EditorSpinSlider` is read-only

(cherry picked from commit 88f1b679f3dd0b66105b29f683fa3dbc0d0686de)

editor/gui/editor_spin_slider.cpp

@@ -664,6 +664,10 @@ bool EditorSpinSlider::is_grabbing() const {
 }
 
 void EditorSpinSlider::_focus_entered() {
+	if (is_read_only()) {
+		return;
+	}
+
 	_ensure_input_popup();
 	value_input->set_text(get_text_value());
 	value_input_popup->set_size(get_size());