Add some IPSLib function to check if an hook exists

[teraßyte]

Right now there is no defined to way check if an hook exists or is enabled unless you manually go through the whole cache and compare everything yourself manually in a foreach cycle, it would be useful to add 2 new functions with those specifics: I have seen quite a few modifications (and a couple of places in IPB too unless I remember wrong?) check each one in their own way if an hook exists in the cache to enable or not specific functions. Also both functions should cache the results in a static variable so that if the same values are checked again there's no additional checking done.

[*]A new function to retrieve an hook by his key (there's no need for an "is enabled" check since the hook won't even be in the cache if it's disabled) [*]A new function to retrieve ALL the hooks executed on a certain location: for example all skin overloaders on skin_global, or all library hooks on classPost, all template hooks on skin_global::globalTemplate, and so on.