Redesign ComboBox filtering, highlighting and selection behaviour.

Redesign ComboBox filtering, highlighting and selection behaviour.
(#15502, #9369)

Changes:
- When opening the popup, the first suggestion is always highlighted by
default unless adding new items is allowed.
- When filter matches currently selected item, that item will be
highlighted instead of the first item.
- Hitting enter or tab will always select the highlighted item.
- Closing the suggestions list by clicking outside the list no longer
selects an item to prevent accidental selections.

Test changes:
- Extended ComboBoxElement to help test filtering.
- Updated and tweaked ComboBoxResetValueTest,
ComboBoxIdenticalItemsTest and ComboboxScrollableWindowTest.
- Added ComboBoxSelectingTest and
ComboBoxSelectingWithNewItemsAllowedTest.
- Updated some tests that were using keyboard navigation.

Change-Id: Ia7745b624bdb0b1a1bb498157ebcb37bee219d76