use KBusyIndicatorWidget for all busy indication uses

Summary:
unlike the pixmapsequence this scales properly and is actually theme color
sensitive (contrast on dark themes) and looks nicely integrated into plasma

Test Plan: triggered all possible busy indicators and they look dandy

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24158