Expose Importer settings

[atteneder]

Problem

At the moment the Editor scripts, including the importers KtxImporter and BasisImporter (along with their base class) are private and thus changing their settings via script is not possible without a load of workarounds.

Solution

All of the settings can be adjusted by both inspector UI and via script.

Idea/feedback from @hybridherbst

[hybridherbst]

cc @pfcDorn so that you're also subscribed here