Add close tab and create new tab on middle click event

Summary:
Click with the middle mouse click on a tab: close it
Click with the middle click on empty portion of tab bar creates a new tab.
This makes konsole more streamlined with default behavior
found in other tabbed applications like browsers.
GUI option to enable/disable closing tab w/ mouse button.  The default
is to have closing tab with button disabled to avoid possible data loss.

Tomaz Canabrava <tcanabrava@kde.org> also coded portion of this.

FEATURE: 398940
FIXED-IN: 19.04
GUI:

Test Plan: {F6618852}

Reviewers: ngraham, hindenburg, tcanabrava

Reviewed By: ngraham, hindenburg

Subscribers: emateli, thsurrel, ngraham, hindenburg, shubham, broulik, konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D15742