Close Dolphin if last tab closed

Close Dolphin if last tab closed

Summary:
Allows closing of last tab via shortcut or menubar (File->Close Tab), which then results in closing dolphin.

FEATURE: 397101

Test Plan: [Manual] Check closing tab closes dolphin via shortcut and menubar.

Reviewers: #dolphin, ngraham, #vdg, elvisangelaccio

Reviewed By: #dolphin, ngraham, #vdg, elvisangelaccio

Subscribers: thsurrel, svenmauch, abetts, elvisangelaccio, ngraham, #dolphin, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D15278