When a combobox expands to retrieve its list of items, it collapses itself after the items have been retrieved.
Added an additional test to ensure that a combobox which was already expanded does not get collapsed after its items are retrieved.
Updated changes log to include the combobox expansion changes.
Improving the Slider API by providing Increment and Decrement methods

This commit improves the Slider API according to the proposal #104.

This adds four methods

Incement()
Increment(double to)
Decrement()
Decrement(double to)