Pass progress to plugin scanner

[grliszas14]

Resolves: audacity/audacity#10719

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[grliszas14]

/build

[saintmatthieu]

nitpick: is the muse:: prefix wanted, since we're already in that namespace? (I don't dislike it, but wanted to point it out. I leave it up to you.)