Make the key binding editor dialog bigger

Summary:
The first column in the key binding editor dialog usually has long lines of
text, to improve usability set a minimum size of 480x430 for that dialog
and an initial size of 500x500. Also set the initial width of that column
to 300.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: ngraham, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D11637

(cherry picked from commit bd560c56)