Add button to swap X and Y in Plot Setup dialog

Summary:
Added the “swap X and Y“ button (icon `swap-panels`) to the Plot Setup dialog in Kalzium. This button swaps X and Y axis.

Screenshot in Czech (see the button between X axis and Y axis):
{F6939022}

Reviewers: #vdg, tcanabrava, #kalzium

Subscribers: filipf, aacid, kde-edu

Tags: #kde_edu, #kalzium

Differential Revision: https://phabricator.kde.org/D22127