UI: improve Goto / Random / End functions and in-process file scan

[fire-eggs]

The background file scanner won't have the full file list until scan is complete.

Need to have useful handling of the Goto and End functions when the scan is in-progress.

E.g. the user tries to Goto image #500 in the list but the file scanner hasn't reached image 500 yet. Or when the user presses the End key to go to the end of the list : at this time, YAIV shows the image at the end of the currently loaded list, not the "actual" end.

Or if the Randomize function is used - a partially shuffled list?

[fire-eggs]

Idea: use the "unverified" list if necessary.

This won't be 100% accurate for Goto / End but should be "close enough".

With randomize, I'm not sure how to handle this. I.e. the randomized "unverified" list will be out-of-sync with the "verified" list, unless extra work is done to re-sync when the file scanner is finished.