Fix #IDEUI-197 ability to style part icon

Add a decorateIcon method to PartPresenter with a default
implementation in AbstractPartPresenter. It is called in addPart
method, both from PartStackPresenter and EditorPartStackPresenter
just after having get the SVGResource from getTitleSVGImage and
before setting up the icon on the part.
Merge pull request #176 from stephanedaviet/IDEUI-197

Fix #IDEUI-197 ability to style part icon