qt: do not use `decodeURIComponent()`

QUrl's decoding/encoding treats special characters such as the delimiter # differently. For example, decodeURIComponent() decodes %23 to #, but when we pass the URL to the core QUrl does not encode # back to %23 (unlike others such as to %20) with the QUrl::FullyEncoded option.

There are other problems with decodeURIComponent(). We need to make sure the originally supplied QUrl/url from the drag event is not decayed into a string, which gets treated differently (intentionally, as we can not assume plain texts to be encoded at all let alone properly) in playlist controller. This is hard to satisfy because JS is not strongly typed.

I think that the fix in !6709 (merged) would already fix what 67e2541b tried to fix.

Close #29091 (closed).

Request review @chub @alexandre-janniaux.