fix: error message when scaling media & invalid media unloading previous

[FelipeDefensor]

@azfoo, I am not entirely sure how the wait_for_signal wrapper works, so take an extra careful look at this one.

The callback triggered by the media duration being available was being called before wait_for_signal when the wrapper resumed the event loop with loop.exec(). As the user takes some time to click the dialog, the wrapper ended up timing out and returning False as a success value, which triggered the error message.

This also somehow takes care of the previous valid media being unloaded when invalid files tried to be loaded.

I didn't find a way to manually test this. The whole media load process is fragile, so I suggest we redo manual tests before releasing.

Closes #440.
Closes #425.

[FelipeDefensor]

Previous failure on CI was due to flaky test.

Note: git commit --allow-empty -m "empty" creates and empty commit that triggers CI.