This will hopefully prevent conflicting routes between multiple plugins from being registered and also ensures a more consistent behaviour compared to BlueprintPlugin mixins.
Thanks to @Salandora for bringing this up.
The new hook allows extending the list of rules for maximum body sizes differing from the default of 100KB and can be used by plugins to allow uploads to them that exceed that file size.
Also extended the plugin manager to detect plugins that implement restart needing hooks (such as the above one) and handling those plugins the same as plugins containing implementations that inherit from octoprint.plugin.core.RestartNeedingPlugin
* moved virtual printer into plugin
* made default serial factory use supplied parameters instead of directly utilizing self._port and similar
* documented new hook
