Add option to have close button on each tab

Summary:
Per VDG, allow closable tabs to have close buttons;
this adds this option while leaving the previous options.

Reviewers: tcanabrava, #vdg, #konsole, ngraham, hindenburg

Reviewed By: tcanabrava, #vdg, #konsole, ngraham, hindenburg

Subscribers: aacid, rizzitello, abetts, ngraham, hindenburg, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17814