Change window title when searching

Summary:
When you search for a file (e.g. "hello world") the window title of Dolphin will now change to "Search for [input]" instead of "baloosearch - /".

BUG: 321575

Test Plan:

• Disables it iself after search bar is hidden
• Works with multiple word searches

Reviewers: Dolphin, ngraham, elvisangelaccio

Reviewed By: Dolphin, ngraham, elvisangelaccio

Subscribers: anthonyfieroni, elvisangelaccio, rkflx, emmanuelp, ngraham, Dolphin

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D8273