[Task Manager] Remove strict URL handling

The code tried hard to ignore garbage URLs, as Qt is quite lenient, e.g. QUrl("Garbage Url") is still valid.
There is no way to change the strictness of a QUrl after creation,
so the code would enforce it by doing QUrl strictUrl(inputUrl.toString(), QUrl::StrictMode).
However, toString() defaults to PrettyDecoded which avoids percent-encoding and keeps spaces
in tact which is not a valid thing to have in a strict URL.
Effectively, we want to ensure a URL is either a valid path to a local file,
or one of the special applications (for menu ids), or preferred for preferred applications, like web browser,

BUG: 385727
FIXED-IN: 5.18.0

Differential Revision: https://phabricator.kde.org/D26941