Import: Asynchronous conversion in dialog

[mkoskim]

When we open Import dialog, it has a preview element (index & content). We should perform this conversion asynchronously. It could also use some more efficient element - e.g. virtualized components - for rendering, because we generally don't want to see all, just if few things are going to be imported correctly.

Current Import dialog:

You see index preview. We have entered "cha" as Chapter prefix (at right), and in the index you can see the result.