Extract poll and finish methods of install progress

[andy128k]

This PR:

• extracts poll method so polling could be done externally (and interspersed with other UI updates if needed).
• adds finish method and use polymorphism instead of preprocessor.
• removes goto by using std::unique_ptr.

[mvo5]

So pkgPackageManager::OrderResult::Incomplete traditionally means that a media-swap (cdrom) is required. Those are of course nowdays extremly rare and maybe we should drop that but AIUI this change would break this support. I tink its fine to just make it bool RInstallProgress::poll()

[andy128k]

Well, does it really matter why the process is "incomplete"? I mean it could be waiting for a next CD/DVD or a next batch downloaded from internet. In both cases reaction is the same -- continue polling.

OTOH If you think that these two states should be separate, then both start and poll methods should be changed to return something else (e.g. std::optional<OrderResult>).

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This PR alone will not help to achieve a separation of installation process and UI. Unfortunately the interface of pkgAcquire is synchronous. I am afraid the best option would be to move the whole process into an own thread and communicate with UI via channels.

[mvo5]

So the issue is that on a real media-swap event the exit code is the same (incomplete) and the UI would assume it is finished and keep polling. But incomplete from the package manager is an end-state, i.e. the loop needs to exit and the dialog with "please insert next disk" needs to come up. its all a bit academic of course as there are no cdrom based distros anymore :)

But I love your suggestions about std::optional - updated the code and pushed here (hope that is okay) and once green will mrege, the refactor itself is very nice.

[mvo5]

Thanks! There is lots to like here, this is some very old (and inelegant) code so a cleanup/refactor is appreciated (but see my inline comment)