luci-mod-system: add led plugin infrastructure

This commit creates the possibility that not only kernel-led-triggers can
be selected but also application-led-triggers from user space.
This is done via a plugin mechanism. The application-led-triggers are scripts
that set kernel-led-triggers on system events or services. Until now this
has not been possible. The package rssileds is a kind of
application-led-trigger.

The following new packages are added:

* luci-app-ledtrig-rssi (application-led-trigger)
* luci-app-ledtrig-switch (kernel-led-trigger) not needed on every most devices
* luci-app-ledtrig-usport (kernel-led-trigger) optional trigger

Since we have now a plugin mechanism I have added the following triggers
as a dependency. So this triggers are now installed per default on LuCI
installation.

* kmod-ledtrig-default-on
* kmod-ledtrig-heartbeat
* kmod-ledtrig-netdev
* kmod-ledtrig-timer

The kernel trigger kmod-ledtrig-usbdev was removed with the commit
https://github.com/openwrt/openwrt/commit/d0b50c2770a0e2d54b37153f2801e2e7dc865fa6
So I have not ported the relevant code anymore.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>