Allow to set-up MapStruct for Java projects through the plug-in

Nobody has claimed this yet.

When working with a Maven project for which MapStruct is enabled (e.g. through via maven-processor-plugin), upon import of the project M2E will automatically configure the Eclipse project to run the MapStruct annotation processor.

But for plain "Java Project"s it'd be nice if the plug-in could be used to set up MapStruct. It could like this:
- Under "Project Properties" have an entry "MapStruct"
- On that page have a check box "Enable MapStruct for this project"; When checked this should register the annotation processor (AFAIK there is an extension point through which APs can be registered)
- Allow to specify the processor options through check boxes as well
- Optionally, have a global preferences pane which can be used to configure default values for the processor options which would be overridden by project-scoped settings

@larswetzer WDYT, would that make sense?

Start by reviewing Eclipse’s annotation-processor registration extension point and how project properties and global preferences are contributed for plain Java projects. Done means the plug-in can enable MapStruct, configure processor options at project scope, and optionally provide defaults that project settings override.

Written by the indexing model from the issue text.