bad textarea width for rename file if filename too short

BUG: 386546

plasma-desktop - bad textarea width in rename file

steps for replicate:

create file (on plasma-destkop) with filename 't1'
try rename file to: 't1 test test'

Reviewers: #plasma, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D10837