Make checkstyle menus available on working sets

[Bananeweizen]

We are using rather large workspaces, grouped by working sets. Unfortunately the checkstyle menu items are only available on projects or project members, not on working sets. I suggest to make these 2 items available on arbitrary working sets:

• Check code with checkstyle
• Remove checkstyle violations

(Arbitrary working sets since it should be possible to iterate over all projects afterwards and silently ignore all non Java projects, so there is no need to spend time to actually calculating the menu visibility based on whether some Java project is contained).