ComboBox: support rounded selection

[DevCharly]

This PR enables using "rounded" selection for combo box popup list.

This is not yet used in any theme, but intended to be used for macOS themes (see PR #533) and future Windows 11 style themes.

Example

UI properties for above screenshot (see Application properties files):

ComboBox.popupInsets = 1,0,1,0
ComboBox.selectionInsets = 0,1,0,1
ComboBox.selectionArc = 6