Bug 1090682 Part 1 - Remove HTMLElement.location usage in selection.py. r=mdas

HTMLElement.location was deprecated. HTMLElement.rect should be used instead.
Bug 1090682 Part 2 - Refactor _long_press_to_select_first_word. r=mdas

Split the function into _first_word_location and _long_press_to_select,
to isolated the side effect which causes focus changed. They can be used
to implement focus changing tests in later patches.
Bug 1090682 Part 3 - Implement focus changing tests. r=mdas