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