Change terminal panel icon to dialog-scripts

Summary: The old icon was a color icon, which does not match the style of other toolbar/menu buttons

Test Plan:

Reviewers: Dolphin, VDG, ngraham

Reviewed By: Dolphin, VDG, ngraham

Subscribers: ngraham, kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D23740