* Added PreferredHeaderWidth() to HeaderRenderer, so that it could add
additional spacing required by the look. Not yet used.
* Added mouse hooks to HeaderView and very humble beginnings of view state
support.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@31886 a95241bf-73f2-0310-859d-f6bbb57e9c96