Data views: add method to retrieve data view type
Retrieve data view type if not passed on data view switch

This change allows to switch to a data view without knowing its type in
advance. The switch method will retrieve it if the type is not passed
in.
Allow data view specification in URL

This commit adds support for URLs like "<catmaid-url>/?dataview=<id>"
where <id> is the primary key for a data view. For now, this works for
all data view types except legacay views. This refers to issue #542.
Add data view links to data view menu

With the help of these links, one can link directly to a data view.
The link symbol is only visible when hovering the menu item. This commit
refers to issue #542.
Merge branch 'dataview-links'