Installables: add uninstaller factory for points type settings

[JDGrimes]

Would be particularly useful because points type settings don't need to be removed when WordPoints itself is going to be uninstalled, because in that case the points types will be deleted entirely. That is a bit of an edge-case, however.

Note that currently there isn't even a function to delete a particular points type setting, but see #314.

[JDGrimes]

Also worth noting that the points functions aren't loaded by default during uninstall.

[JDGrimes]

The biggest issue here is that these functions will end up calling is_wordpoints_network_active() when retrieving the option. Which gives a doing-it-wrong error during uninstall. To avoid that, we'd have to retrieve the option directly. So that's why this would need to be a part of core, because in case we change how points type settings are stored in the future.

[JDGrimes]

One problem with extensions that would like to use this is that the extensions are uninstalled before the components are. So the autoloading for the points component, where this class ought to be stored, won't be set up yet. This would mean that it would need to be included manually. That seems kind of silly though, since when WordPoints as a whole is being uninstalled, we don't need to run this routine anyway. (And when only the extension is being installed, the points component will probably be loaded already. This isn't guaranteed, however, since the points component can be deactivated.)

These are general issues with uninstallers being provided by components, that we haven't really thought about yet, I guess.

Just setting up autoloading for the classes directories for the components, or just including the file specifically, won't necessarily be sufficient in every case either, since the uninstallers may also use other component functions. Extensions will either need to always check if these uninstallers exist before using them, or we will need to load the components entirely whenever an extension is being uninstalled.

I guess this really relates to dependency management. In cases like this, an extension's dependencies need to be loaded when it is uninstalled. Which is something I guess we haven't really thought much about.

[JDGrimes]

That is a question that I really don't want to tackle this close to release. But even if we punt this ticket, I guess we've already raised the question with the points hooks uninstaller factory. At present we do use that factory internally though, so it isn't just intended for extensions. But we did intend that extensions could use it also.

Perhaps we just shouldn't over-complicate things right now. Neither of these uninstallers are going to be dependent on the component's functions (at least not right now), and so at present it is safe to include/autoload them. Of course it is possible for that to change in the future, which is why we'd prefer to resolve the larger issue, but perhaps we just shouldn't tackle it.

[JDGrimes]

Let's punt this for now, and wait to introduce this until we've resolved this problem.