hints when using the select by annotation dialog box

When using the select by annotation box, it is not obvious that one of the check boxes needs to be selected before Jalview will actually filter the columns according to annotation.

1. A message needs to be added to tell the user that one of the check boxes needs to be selected in order for a search filter to activate, and similarly for the structures filter.

2. Since this dialog only selects columns, currently, it should also be called 'Select columns by annotation'.