Viewer loading bar should be determinate & better placed

[didimitrie]

Step 0:

• I've read the contribution guidelines!

The viewer's progress bar is currently indeterminate, even if it is actually a determinate process (number of object request buckets left).

Flagging this up as it's rather stupid not to properly display model loading progress when we actually can (consider adding a percentage info somewhere too, as the bar itself is rather abstract).