Restore ability to close tab by clicking on close button

Summary:
After making KTabBar and KTabWidget part of Konqueror, clicking on the close
button of a tab didn't make the tab close any longer. This fixes it

Test Plan:
Open several tabs in Konqueror, then click on their close button and
check that they close. Check that when only one tab remains, the tab bar is
hidden

Reviewers: dfaure

Reviewed By: dfaure

Differential Revision: https://phabricator.kde.org/D25662