The two new widgets are pretty trivial now, thanks to the base
class.
Note there is still no code to delete the widgets. That's also
why you will always see a placeholder widget at the top. This
will be fixed with the next patches.
This patch also renames most of the "…TemplateOutline…" classes
to "…TransclusionOutline…" The reason is that these widgets are
not for a single template, but part of the container widget for
a more complex transclusion (i.e. a sequence of multiple
templates and wikitext snippets).
Bug: T274544
Change-Id: If4219b0b8ad4d1969ab1ec5ec4db0728811bab35