Themes should not enforce min sizes on blocks in continuous mode; in
this case, the filled block should be as large as it needs to be to
reflect the current value, and no larger or smaller than that. So, the
fact that the minimal size was selected on just levelbar block is wrong:
we should also require the levelbar.discrete class to apply min sizes.

The widget should enforce whatever correct minimum size results from the
above fix, by reapplying commit 78b4885f

Except: we should not allocate/draw the filled block if the value is 0,
as in this case, the LevelBar should be empty, not have a min-size fill.

This partially reverts commit 96062ffe,
as it makes sense to set min sizes for discrete blocks, so keep that in.

https://bugzilla.gnome.org/show_bug.cgi?id=783649