Add warning for ExecuteAlways on wrong target

[citizenmatt]

Unity has several attributes that do not specify a target, but are only used on a specific entity - e.g. [SerializedField] is only applicable to fields, but can be applied to classes and methods. We have an inspection for this to show a warning if an attribute is applied to the wrong target. Add support for [ExecuteAlways] (introduced in 2018.3). It should be applied to classes only.

[citizenmatt]

Have a quick pass through the API to see if there are any other attributes that don't have targets