Improve QDialogButtonBox::Yes override text

Summary:
We are asking the users whether they are sure they want to quit.

But the "Quit" button can be ambiguous (am I quitting dolphin or just
this dialog?). If we use a "Quit Dolphin" button instead, that clearly
shows what will happen after triggering the action.

Test Plan: Close dolphin while multiple tabs are open.

Reviewers: #dolphin

Differential Revision: https://phabricator.kde.org/D11892