Don't use system style for progress bar on Linux (Qt5)

Fusion works reasonably well with the common styles, so use it to
have a predictable result with every style.

For example Breeze shows percentage next to the progressbar (making
their length dependant on the current progress) and use the color of
highlighted row for the progressbar, making them disappear completely
now that we are coloring the background.

Issue #2166.