Give the user more feedback when importing a private key

[the-metalworker]

When a private key is imported, a box pops up that says "The import process for the private keys has started. This will take a long time, up to 6 hours! Please be patient!" with a button that says "Close".

After clicking the "Close" button it's not obvious that anything is happening. I can't tell whether clicking "Close" stopped the private key import, or if the import is still happening, or what.

It'd be nice if the wallet gave the user some more feedback about what is going on here, maybe a progress bar somewhere or something letting the user know that the private key import is ongoing and an estimation of how close it is to being done.