up to now we were only updating the picker selection when the picker
was created, which means that subsequent changes in the text field were
not propagated to the drop-down list
This patch creates a private syncSelection() method which is called each time
the picker is shown
This is roughly based on the ExtJS 4 ComboBox behaviour
The IPrefSelector ComboGrid can have selected values which are not backed
by the component store, ie the store only contains IP aliases, but
the ComboGrid can contain an IP adress not registered as an IP alias.
In that case we should not try to update the selection in the dropdown,
as the dropdown only knows about the component store.
Implementation is similar to the syncSelection() private method of the
ExtJS4 ComboBox.
Instead of extending the framework ComboBox, we extend the abstract
parent class Picker, so we don't rely on private methods ( and hopefull survive the
next ExtJS upgrade )
Usability improvement:
The selection model is now a checkboxmodel, which makes more clear to the user that
we can select more than one element in the list.
The Picker of the ComboBox, has by default a maxHeight of 300 px. If the list of elements is bigger that 300 pixels, then a scrollbar will be added.
This behaviour was broken with ExtJS4, but now works correctly with ExtJS6.
