added error popup when an asset is imported but aseprite path is improperly configured

[edwardrowe]

This addresses the issue I submitted #28.

Some info about my change:

• The Popup assumes that if the Importer.IsConfigured() test fails, it's because Aseprite wasn't linked. This might be a problem in the future if other import plugins require alternative tests.
• The only elegant way I could see to test for the file was to add the IsConfigured function to the IAnimationImporterPlugin. Please feel free to rework this or suggest another change.
• There could easily be some other things I didn't consider with .pyxel files. I did not test this with PyxelEdit
• You may not like the warning HelpBox when Aseprite path is not setup, as some users just won't use Aseprite. I made it a Warning (not error) for this reason, but it still may be intrusive.

Here's a gif of the new behavior.

[edwardrowe]

This in no way blocks us, I just felt like contributing back to the repository for #hacktoberfest =) (https://hacktoberfest.digitalocean.com/)

[edwardrowe]

Thinking this should actually throw an exception and be caught on the AnimationImporter Window. This is because we use our own importer that uses an AnimationImporter (and circumvents the window), and we'd want to be able to popup a window there, too, if you drop a file before configuring the Aseprite settings.

I'll try to rework it soon.

[talecrafter]

The exception might be the way to go, but for now at least the warning is a huge improvement over the current situation. Thank you.