Split jquery.textSelection and WikiEditor api overrides

Split context.evt up into 2 objects
- context.fn, where WikiEditor API functions are defined
- textSelectionFn, where all textSelection API function are defined.

- scrollToTop() is moved to the WikiEditor API block
- getElementAtCursor() is removed. It is no longer in use by the
WikiEditor API or the textSelection API.

This depends on MediaWiki core change I14492572

Bug: 29328
Change-Id: I57bc7791dc61b7edcf258e6c16445d5a4a8783cf