UI: align W axis marker/home buttons with the X/Y/Z columns

The xyzabc rows have three actions (set-position cog, zero marker, home),
W only has two. Without a placeholder the W buttons render in the left two
slots of the actions cell, leaving the home button unaligned with the home
column above. Added a hidden disabled cog button so the marker and home
buttons sit under the same columns as the rest.

src/pug/templates/control-view.pug

@@ -206,6 +206,13 @@ script#control-view-template(type="text/x-template")
                 | {{w.tstate}}
 
               th.actions
+                // Invisible placeholder so the W marker/home buttons line
+                // up under the same columns as the X/Y/Z buttons above
+                // (which have a third "set position" cog).
+                button.pure-button(disabled,
+                  style="height:60px;width:60px;visibility:hidden")
+                  .fa.fa-cog
+
                 button.pure-button(title="Set W axis position to 0.",
                   @click="aux_set_zero()", style="height:60px;width:60px")
                   .fa.fa-map-marker