webadmin: Added infrastructure for linking to System Tree items

This will facilitate adding links to tabs and subtabs, which when
pressed will select the corresponding item in the System Tree.

* LinkColumnWithTooltip is used to display text in a CellTable column
  as a link, such that upon clicking a supplied FieldUpdater is
  triggered.

* SystemTreeModel is modified to enable easy entity lookup by ID. To
  enable access to a SystemTreeItemModel, for example to be accessed
  by a link, one should add it to the member map treeItemById.

* SystemTreeProviderModel now has a setSelectedItem() method that
  receives a GUID, and then looks up the corresponding
  SystemTreeItemModel using that GUID and sets that model as selected.

* A convenience method is added to SystemTree to expand the whole path
  from the root of the tree to a target node, rather than expanding the
  whole tree up to a certain level.

* Small workaround is used to bypass a GWT CellTree issue.

Link to GWT CellTree issue:
http://code.google.com/p/google-web-toolkit/issues/detail?id=6310

Change-Id: I799aa4dfcd8ad7264057810e408cd1c64ea711e8
Signed-off-by: Lior Vernia <lvernia@redhat.com>