Deprecate MarkupContainer#get(int) and MarkupContainer#swap(int, int)

[Martijn Dashorst]

All,

We'd like to deprecate and mark for deletion in wicket 8 the following methods:

 - MarkupContainer#get(int), and
 - MarkupContainer#swap(int, int)

Both methods are not used inside Wicket, and have little utility
outside Wicket, as far as Emond and I could argue, but expose/limit
the internal behavior of MarkupContainer.

Martijn