Introduce configuration option that would define the behaviour of the ComboBox / DropDownList upon item selection after filtering

[veselints]

Introduce configuration option that would define the behaviour of the ComboBox / DropDownList upon item selection after filtering.

Here is the scenario concerned, reproducible on the Server filtering demo of the ComboBox:

• Type 'que' in the widget input;
• Select one of the options - the drop-down will close;
• Click the arrow button to open the drop-down;
• The full list of items will be displayed in the drop-down;

In the above scenario, the input of the ComboBox (which is also its filter field) will be populated with the text of the selected item. That would be a mismatch with the items displayed in the drop-down (all items). A configuration property of the widget could control that behaviour. Depending on the option chosen, it will preserve the current behavior (displaying all items), or it will display only the items matching the selected item text.

In the case of the DropDownList, the current behaviour clears its filter upon selection and renders all available items in the pop-up, when it is opened. The same configuration property could force it to preserve the filter and display the previously filtered items (as the DropDownTree behaves currently).

This is related to #2950, #3965, #3560, and https://github.com/telerik/kendo/issues/9001