luci-base: luci.js: emit custom events for tooltip open/close actions

The new `tooltip-open` and `tooltip-close` events allow other code to hook
into the tooltip div rendering, e.g. to populate it with custom markup.

Also ignore tooltip events originating from descendant elements while
we're at it.

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