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

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?

首先檢查 Eclipse 的 annotation-processor 註冊擴充點，以及如何為一般 Java 專案提供專案屬性和全域偏好設定。完成的標準是：該外掛程式能夠啟用 MapStruct，在專案範圍內設定處理器選項，並可選擇提供預設值，且專案設定能夠覆寫這些預設值。