[Bindings] Extend part scenarios and test

The 'part' binding handler which allows 'transcluding' elements into
binding handler templates, as used by the pager, has been extended to
support more scenarios and now has its own tests
[Tests] Do not hide fixture for hidden DOM tests

To properly test whether a node is visible or not the fixture needs to
be
visible so properties of a node can be checked (e.g. visible width /
height),
not relying on that node itself having display:none or similar
[Bindings] Remove data-option

data-option attribute will be removed to clear clutter when debugging,
as the value is transposed into a data-bind attribute
[UI] Start of an expandable panel control