Support adding metadata to most asset types

[Eideren]

Changed the button from being implemented as an editor to part of the inspector header to support any asset types out of the box

While the button is visible on most assets, adding metadata to some types is not fully supported; scenes, prefabs, textures and models remain unaffected or throw with the following:

Failed to load 'ASSET_PATH'. File may be corrupted or was serialized with a newer version of Unity.
UnityEditor.AssetDatabase:AddObjectToAsset (UnityEngine.Object,string)
AssetMetadataUtility:Add (UnityEngine.Object,System.Type) (at ./Packages/CustomAssetMetadata/Editor/Common/AssetMetadataUtility.cs:169)

[Eideren]

Note that supporting all asset types is possible, but includes a fairly harsh workaround forcing us to append an id to the end of the name of all assets with metadata, it's done automatically on build, but it's still a bit too invasive for wide use I would think.
I could not find a better work around than this one sadly, if we had a fixed id we could retrieve for built assets this wouldn't need to exist.
See Eideren@881b928 for an example implementation of that.