Change drop indicator color

Summary:
Change drop indicator color form highlight to text.
This makes more visible, when indicator is adjacent to a highlight
item, of a list.

BUG: 415010

Test Plan:
Before vs After

Reviewers: Dolphin, VDG, elvisangelaccio, ngraham

Reviewed By: Dolphin, VDG, ngraham

Subscribers: meven, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D26936