AxisPositionHandler: remove superfluous priority
SystemInformation: dump all available plugins

The plugins are grouped by type, alphabetized by fully qualified class.

Also, since services are a type of plugin, we no longer need to
explicitly dump the services.
Avoid overspecifying plugin type

This makes it easier to understand (for both humans and machines) that a
particular plugin belongs to a certain class of plugins. In particular,
the System Information command can more easily group them together as
expected, without reporting many single-plugin types.

This also has the advantage of not loading the plugin classes themselves
until they actually need to be instantiated; SezPoz only needs to load
the plugin *types*, not the plugin *implementations*.
LegacyPluginFinder: make legacy plugins a subtype

They used to be typed as Command.class directly, but it is nice to type
them on LegacyCommand.class instead so that they can be easily
differentiated. In particular, it allows the System Information command
to group them separately.