Support for automatic reimporting

[tullrich]

This pull request add support for automatic reimporting of .aseprite and .ase assets whenever the editor regains focus. "Automatic Reimport" must be enabled in the ImportSettings object for this feature to work on a particular file.

I've made the following source changes:

• Added AnimationAssetPostprocessor which is an implementation of Unity's AssetPostprocessor in order to detect asset changes and call AseImporter.Import() as appropriate.
• Refactored a few of the private static functions previously found in ImportMenu.cs into public functions found in ImportSettingsReference.cs for easier access (felt this was the most appropriate location).
• Added an "automaticReimport" boolean to the ImportSettings object. Currently this is defaulted to false. This must be set to true in order for assets referencing this ImportSettings to be reimported.

This hasn't been super thoroughly tested yet. However, much of this is based on how Animation Importer does the same, so I don't anticipate major problems.

I'll be happy to make any changes you want. Thanks!

[WeAthFoLD]

It seems more approriate that default value of automaticImport can be configured globally, or even become a global setting (some projects will likely want all ase to be automatically imported). Apart from that, the reimport feature itself looks pretty solid. I'll think about it more and then make the decision about the location of the setting.

Thanks a lot for your contribution!