[ComboBox] Close dropdown on mouse up instead of mouse down.

[jpobst]

Fixes #87

The ListBox used in a ComboBox raises its SelectedIndexChanged on MouseDown, which closes the ComboBox dropdown. This causes the subsequent MouseUp event to be processed by the Form/Control that was underneath the ComboBox which is not desired.

Allow the ComboBox to set an internal property on the ListBox that tells it to change the SelectedIndexChanged event on MouseUp instead. This appears to match Winforms behavior better as well.