Cleanup eclipse formatter settings

Sync the explicit setting with 'project specific settings' in case
the eclipse plugin is not used.
Various improvements to the Gradle plugin

Refactor and rework several areas of the Gradle plugin:

- Refactor package structure into functional areas and configure each
  area separately via a new PluginFeatures interface.
- Convert BootRunTask to extend RunTask rather than attempting to
  find existing RunTasks.
- Simplify agent integration by no longer needing specific BootRunTask
  code.
- Update the repackage task to consider the `mainClassName` property
  in addition to `springBoot.mainClass`.
- Automatically set `mainClassName` when calling `run` or `runBoot`
  from `springBoot.mainClass` if there is one.
- Ensure that explicitly defined `main` options on JavaExec tasks always
  take precedence.

Fixes gh-547, gh-820, gh-886, gh-912