Overwrite existing asset import settings

[hiroki-o]

Original report by thomas231 (Bitbucket: thomas231, GitHub: thomas231).

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We have several assets that do not go into an asset bundle. For those, we have a dedicated build script to set the correct import settings like sprite atlas tags.
it would be great if ABHT could also be used to handle this task. We would just need a way to overwrite the meta files for assets in the projects. Currently, we can only create new assets.

[hiroki-o]

Original comment by Hiroki Omae (Bitbucket: [Hiroki Omae](https://bitbucket.org/Hiroki Omae), ).

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You can already do this by creating dedicated Loader and ImportSetting node for this purpose, and pass those assets you wish to overwrite import settings to it, and just do not use the output of ImportSetting any further.

On version 1.2 ABGT will support multiple graphs, which will make easy to use ABGT for creating pipelines of general automation purposes.

[hiroki-o]

Original comment by Hiroki Omae (Bitbucket: [Hiroki Omae](https://bitbucket.org/Hiroki Omae), ).

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Multiple graph support just landed onto dev branch. You can now create graph with just Loader and ImportSetting to do these works, aside from asset bundle configuration.

https://trello.com/c/xTT1zTyQ/135-multiple-graph-support

AssetBundleGraphUtility is added to execute specific graph from script. You can create graph to do your work and add small code to call it from menu just like this:

#!c#

	[UnityEditor.MenuItem("TestMenu/Force Import Settings")]
	public static void ForceImportSettings() {
		UnityEngine.AssetBundles.GraphTool.AssetBundleGraphUtility.ExecuteGraph("Assets/ForceImportSetting.asset");
	}