Running import/export channel operations within a task for import/export content operations does not properly manage the overall progress

[indirectlylit]

Observed behavior

when exporting content to disk, progress hangs on 100%, resets to 0, and then starts over

Expected behavior

The entire process of exporting should be represented by a single pass of 0-100%, because the user is not aware of sub-stages

User-facing consequences

confusion & possible annoyance. Risk of hitting 'cancel' and losing progress

Steps to reproduce

• select content to export to disk
• begin process
• watch bar get to 100%, hang there, start over, and then reach 100% again

Context

0.12.3

[pcenov]

@radinamatic this one is still extant in 0.15.1 although in a slightly different way - once I hit the Export button I see the progress bar reaching 84% then goes to 9% and then proceeds to 100%:

https://user-images.githubusercontent.com/79847249/159944869-24439047-4f06-4c98-a771-d2ff09d824ec.mp4

[rtibbles]

I am guessing this is probably due to a mismatch between progress reporting of the component jobs (exportchannel and exportcontent) that make up the overall flow here.