Fixing issue with Vaadin 7 combobox when typing and tabing out fast (#12033)

Fixing issue with Vaadin 7 combobox when typing and tabing out fast (#12033)

ComboBox does not select or add a new value (in case allowed) if User enters the value by typing and TABs out fast. This bug was originally reported in https://github.com/vaadin/framework/issues/4276 and fixed by https://dev.vaadin.com/review/#/c/3564/6/client/src/com/vaadin/client/ui/VFilterSelect.java. However later some logic of VFilterSelect was refactored by patch https://github.com/vaadin/framework/commit/acb889336f80227d609b194e56ac6ae3ead0d338, which accidentally removed the fix. In this new PR I am re-applying the missing part of the logic (adopted to the new structure)

Fixes https://github.com/vaadin/framework/issues/6671