Refactoring: startmode_for_product -> new_device_startmode

... and code cleanup

Most significant change in functionality is that default startmode is
handled completely in this function. It used to be solved partially
outside (when default startmode by product wasn't in {"auto",
"ifplugd"}).
Adapted testsuite to changes caused by refactoring.
Do not display Add button in Overview dialog during installation.

User should be able to configure existing devices during installation.
This can be achieved via "Edit" button on automatically detected
devices. Advanced configuration should be postponed to installed system.
Updated changelog