implement localized tabindex ordering

[rodneyrehm]

using the technique "dynamic tabindex reallocation" [1] explored in https://jsbin.com/gumexa/10/edit?html,js,output it is possible to define the sequential navigation order constricted to groups of elements (rather than globally for the entire document).

[1] I suck at naming things

[rodneyrehm]

allowing to group elements this way would make it possible for elements that are not part of the group to not ever receive focus:

<input data-focus-group="gustav">
<input> <!--  not be accessible to keyboard focus-->
<input data-focus-group="gustav">

[rodneyrehm]

redirecting focus to first/last element upon entering the group must be limited to the keyboard, as mouse (and script) may be used for "random access"