Fix encoding of size tooltip in radial map
class File (see fileTree.h) internally stores filenames in
8 bit encoding. The size tooltip uses the same API, but does
not correctly encode the tooltip to 8 bit. It uses UTF-8,
while the system might have a different local encoding.
Changing toUtf8() to QFile::encodeName() fixes this.
BUG: 411243
FIXED-IN: 19.08.2
Reviewed by: aacid
Differential Revision: https://phabricator.kde.org/D24013