luci-base: add clone action for tables

luci-base: add clone action for tables

This augments CBITableSection, affecting types which extend it, i.e.
CBIGridSection.

Setting a table 'cloneable' property to true reveals a column of clone
buttons who designate the current entry as a clone source. Clicking the
clone button duplicates the data of that section_id into a new entry,
while the new entry gets a new and unique SID. E.g.

	s = m.section(form.GridSection, 'foo', _('Bar'));
	...
	s.cloneable = true;

Clone and add actions differ: clone will not open a dialogue. That is a
user exercise.

One may set the put_next flag to false to put the new clone last, or
true to put it next (after the clone source).

This uses a new uci action which fulfills the behaviour: clone

It is possible for the uci clone action to be used independently.

See also:
https://forum.openwrt.org/t/add-clone-button-to-luci-configurations-esp-in-firewall/196232

Signed-off-by: Paul Donald <newtwen+github@gmail.com>