[enhancement] allow configuration of generated Java code indentation

One of the most attractive features of Xtend is that it generates Java source code, as it makes the transition from Java easier, it is useful to understand the semantics of Xtend, and it is key when working in projects with developers who are familiar with Java but not with Xtend. Unfortunately, the formatting of the generated Java code is less than ideal, and what's worse, it cannot be changed.

One problem particularly sticks out: the generated code uses a default indentation of two spaces, which can be quite hard to read (specially for people with poor eyesight). It would be desirable to be able to specify the indentation size in the Xtend preferences and maybe even the type: tabs or spaces.

Even better, generated Java code should adhere to whatever formatting settings are defined in the JDT. (But for now I would be happy if at least the indentation issue is resolved ;))

Start by locating the Xtend Java code-generation and preference entry points; no specific files or tests are named in the issue. The work is done when generated Java indentation can be configured for size and, if supported, tabs or spaces, with the behavior covered by the relevant existing tests.