Reduce the size of busy indicators

Summary:
As reported in 406940, the busy indicators that show up when data is being loaded are a bit too big at the moment. These changes reduce their size by anchoring them at the center instead of having them fill the available space (see example screenshots below for comparison).

BUG: 406940

Track view before change:

Track view after change:

Grid view before change:

Grid view after change:

Test Plan: This is just a minor UI tweak. No behavioural changes.

Reviewers: Elisa, mgallien, VDG, ngraham

Reviewed By: Elisa, mgallien, VDG, ngraham

Subscribers: ngraham

Tags: Elisa

Differential Revision: https://phabricator.kde.org/D20880