First full commit of plugin system

Should work fine this.

I've put a test plugin which you can use to verify the system is working on my GitHub, https://github.com/laf/Test

If you want to clone the that into a librenms repo so it looks more official then no probs.

At the moment this will only integrate into the Plugin system menu and has no other hooks available but these are easy to add in. However full php can be used within the plugin page itself.

This commit has the removal of the widescreen link as this is not active and doesn't work so it's been replaced with the Plugins link instead.

See #155 for previous comments.