TMF: Refactor XML model code, using factories to re-use element parsers

TMF: Refactor XML model code, using factories to re-use element parsers

The same XML syntax (and thus parsing) can be used for many XML components'
element, like state providers and views, but the behavior of each component
may be different. We use a factory to define create the classes with the
various behaviors.

Change-Id: Iec5d54155b93a79b1ebc5ffeb8eaffef5e6569ca
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/23157
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>