In detail:
* Allow clicks on all elements in the new sidebar. This should
focus the corresponding element on the right.
* Make all elements in the new sidebar tabbable.
* Fix MWTransclusionOutlineTemplateWidget.createCheckbox() to
not need a temporary param object any more.
* Rewrite more code in MWTransclusionOutlineTemplateWidget to
be shorter and easier to read.
* Fix MWTemplateModel.addParameter() to not do way to much
stuff when a parameter already exists.
* Update code documentation.
* Use more specific, less ambiguous variable and method names.
Bug: T274544
Change-Id: Iaf6f7d1b0f7bf0e9b03eb86d01f3eceadece6fe4