[TagDialog] Really enable "Open in filemanager" button for local files
QUrl::QUrl() expects a full Url including the scheme (unlike
KUrl::KUrl() that supports local paths too).
But for local files, urlString contains the local path without a
scheme ("file://").
The problem here is that QUrl cannot detect that the Url is actually
pointing to a local file if the scheme is missing (according to the Qt
docs, a URL is a local file path if the scheme is "file"), so
QUrl::isLocalFile() was always false and the "Open in filemanager"
button never got enabled.
So use QUrl::fromUserInput() instead to construct the QUrl, which
also supports local paths.
Also, to avoid showing the "file://" scheme in the "Location" text field
now, pass QUrl::PreferLocalFile to QUrl::toDisplayString() as
suggested in the KUrl::pathOrUrl() docs.
Differential Revision: https://phabricator.kde.org/D24043