When the selection is deselected, restart the keyboard search from the beginning

Summary:
BUG: 411538
FIXED-IN: 19.12

Test Plan:
Open a directory with 3 files starting with the same letter.

1. Press this letter key twice
2. The second file is selected
3. Deselect the file with the mouse or using Esc
4. Wait 1 second
5. Press the same key again

Before:
The third file gets selected

After:
The first file get selected

ctest

Reviewers: Dolphin, elvisangelaccio, ngraham

Reviewed By: Dolphin, elvisangelaccio, ngraham

Subscribers: ngraham, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D23716