Add Ability to Hide Icons for CS Info Level

[SoerenHenning]

Currently, its only possible to either hide all checkstyle label decorations or to show all. It would be nice to only display errors and warnings but to hide the info icons. Otherwise, often all files are labeled due to missing JavaDoc properties.

[ChristianWulf]

Use the filter level of the filter element of the violations view.

[ChristianWulf]

Please verify. If ok, please close this issue.

[SoerenHenning]

This is implemented for the editor now, but what I was referring to was hiding the icons in the "Package Explorer" view. For the editor, this feature might be nice as well, but for the list of files it is even more useful. (For instance because of the example described above.)