Statusbar progress widget: update up/down icon when dialog goes away

Summary:
The two need to always be in sync. The button was still showing "down"
when the dialog went away with a timer.

Since slotToggleVisibility() ends up calling StatusbarProgressWidget::slotProgressDialogVisible
slotProgressButtonClicked() doesn't need to updateProgressButton() directly.

Test Plan: Looking at the button at the end of a sync

Reviewers: mlaurent, dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Differential Revision: https://phabricator.kde.org/D21067