Add command for running checkstyle

[Bananeweizen]

This adds a command for the already existing action to run checkstyle on the selected resources. That way users can invoke the command also via Ctrl-3 quick assist or via a key binding.

There is no default key binding, as the normal workflow for users is to rely on the Eclipse builder and to not invoke this command manually.

fixes #99

Be aware the shown keybinding is not available by default. I just added that for testing and to illustrate that a keybinding can be given by the user.