window-list: Don't use panel-button class for the workspace indicator

The panel-button introduces some horizontal padding which is insensitive
to scroll events. Without this change, there is a small dead zone in the
corner that cannot be used to switch workspaces with the mouse wheel.

For useMenu mode, this has the effect of removing all of the horizontal
space to the edge of the screen, so I add some back with the
status-label-bin margin.

This a is similar change to 8bad8a3b.

Fixes #315.

Part-of: <!171>
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