luci-base: ui.js: implement AbstractElement.isChanged()
luci-base: ui.js: implement AbstractElement.isChanged()

The new `isChanged()` utility functions allows to query the dirty state
of an ui input widget.

Signed-off-by: Jo-Philipp Wich <bd73d35759d75cc215150d1bbc94f1b1078bee01@mein.io>

openwrt/luci,openwrt/luci,hnyman/luci,hnyman/luci,hnyman/luci,hnyman/luci,openwrt/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,hnyman/luci,hnyman/luci,openwrt/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci,openwrt/luci,openwrt/luci,openwrt/luci,hnyman/luci,lbthomsen/openwrt-luci,lbthomsen/openwrt-luci