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