mediawiki.Title: Fix doc reference error

Follows-up 165373d which introduced a reference to a browser
global not in our index (causing a jsduck error).

Unless we're using an element that has special DOM methods I
don't think it's worth adding element subclasses to the doc index.
We generally just use HTMLElement so changed it to that.

Also made a minor optimisation in merging the img/src assignments
by using .prop( 'src' ) and .src directly. And while at it also
reduced it to direct access with [0] instead of .get().

Change-Id: I3a521eda0aeeb7847a96d474316d1d5e6e8917d1