qt: do not fully collapse the artists pane due to small window width

When there is not enough width, we can collapse to just show the cover art. This is also the planned behavior of the upcoming side- bar (showing an icon when available, or abbreviation, instead of fully collapsing).

Only showing the cover art requires less than 50dp width. Requiring such small space makes full collapsing hard to justify.

Fully collapsing things are incredibly bad for UX, especially when there is no alternative to what the collapsed thing provides. In this case, the user has no option changing the current item when the artists pane is fully collapsed.

It looks like this: