Add getter/setter for the next custom color index in QColorDialog.

Currently custom colors can be set/retrieved, but not the index of the next custom color to be set. This value is set to 0 in the dialog's initialization, which means every time the dialog is opened user can either overwrite the first custom color or select another slot first, which changes the currently selected color. Both of these options are equally annoying to the users.

It would be thus beneficial to be able to save/restore that value. It is already tracked internally (QColorDialogPrivate::nextCust), so it's just a matter of adding a public getter/setter.

Related proposal is that the value could be stored/restored together with the custom colors themselves, or at least made static, relieving the user from having to manually store/restore it every time. Since the custom colors are shared between dialog invocations that index should be too.