Improve keyboard controls of the checksum widget

Summary:
Previously when using the tab key to navigate the widget,
it would select the calculate buttons in the order:
SHA1->SHA256->MD5
This patch moves MD5 to be the first selection to match the visuals

Test Plan: Manual testing on several files.

Reviewers: VDG, elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: elvisangelaccio, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18676