Update thumbnail de-/select hover button on selection change

Summary:
The de-/select (-/+) hover button of the thumbnails in Browse
Mode or thumbnail bar in View Mode does not reflect the actual
selection state sometimes.
This patch updates the button state when the selection changed.

BUG: 394406
FIXED-IN: 18.04.2

Test Plan:

1. Open Gwenview in Browse Mode or View Mode with visible thumbnail bar
2. Use e.g. Ctrl+Click to de-/select images or move selection by keyboard
3. -/+ button of item under mouse pointer should reflect the actual selection state

Reviewers: Gwenview, huoni

Reviewed By: Gwenview, huoni

Subscribers: rkflx

Differential Revision: https://phabricator.kde.org/D13010