Export engine improvements

[fcamarlinghi]

• Support for background matte color when exporting groups with transparent backgrounds.
• Give more control over the blur/sharpen settings.
• Additional scaling options (useful i.e. to export textures for mobile games), including support for specifying the output resolution in pixels.
• Auto-export enabled targets when changes are made to the document.
• Support to optionally save settings to an external file instead of using document XMP data.
• Support for running a post-export step (i.e. a script to copy the exported file to a different folder, see multiple export paths #8 ).

[fcamarlinghi]

• Support using PSD alpha channels for alpha.

The only issue I have with the Alpha-Folder: If some other team-members don't use the plugin, they can not save the textures correctly the normal way (file --> save as). They might see an "alpha"-folder but don't know, that they would have to copy the content of it into the alpha channel before exporting. Of course, I could to that but if I forget once, it could lead to weird issues as soon as someone else exports this texture (manually).
Maybe an idea would be to add an option "use alpha channel" in the drop down for the alpha?