Stack: add control to toggle slice controls

Adds a button in the lower left of the stack view, similar to the
overview toggle, that toggles visibility of the layer sliders and scale
benchmark. See discussion in acardona/CATMAID#434.
Revert "Tracing layer: add button to toggle slice controls"

This reverts commit 957331916f33c89e01f77a408a252775d0bdc31d.
Stack: fix slice control toggle pointer CSS (m)
Stack: better stack control toggle icon

Replace the stack control toggle icon with one from @tomka more
consistent with other stack UI and indicating toggle state.
Merge branch 'issues/434-slice-control-toggle'