Added screen resource with provider, supporting the usual actions...

[nightw]

Also added the ability to looking up the resources for the screenitems (so you don't have to know the resourceids for the screen items, which are dynamic by nature, so you would not know anyway)

[guilhem]

@nightw can you add some tests in test/kitchen/cookbooks/zabbix_test/recipes for your LWRPs

I don't want to break things in future updates.