luci-base: move DOM manipulation functions to luci.js

Introduce a new luci.dom class which groups the DOM manipulation helpers
such as E(), findParent(), matchesElem() etc.

Provide wrappers for the old functions to ease the transition to the new
functions.

Also add a new widget helper function L.itemlist() which consolidates
the item enumeration formatting code found on various pages.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>