Improve the link browser

One of the most important tasks in XWiki is to link content in XWiki. At present the menu option for creating links provide a link browser for other sites and one for attachments.

But often if you would refer to another site in XWiki you would refer to a headline or manually set anchor. Therefore the link browser could be improved.

At first the browsing for sites and for attachments could be merged. The attachment link browser differs just in additional symbols for attachments. Second the link browser should also offer headline links and links to manually set anchors.

The link syntax in xwiki code is not reasonable for standard users. But they should be able to interconnect their content.