Support 'style only' override in style system

[Garry Yao]

This's a ticket derived from #705, which provide a UC of the following style definition which we don't support now:

{ element : 'strong', overrides : [ { element: 'b'}, { element: 'font', styles : { 'font-weight' : 'bold' } } ] }

And a more generic UC ( and actually a common case )with 'style' only definition could be inferred as:

{ element : 'strong', overrides : [ { element: 'b'}, { element: '*', styles : { 'font-weight' : 'bold' } } ] }

Which declare that we need to consider bold for the following two cases:

1. It's a <strong> tag;
2. It's a <b> tag;
3. It's any tag with "font-weight" : "bold" style;

[Frederico Caldeira Knabben]

Milestone CKEditor 3.x deleted

[Jakub Ś]

This ticket is very similar to #5980 however I think that #5980 sounds better.