Use OP tooltips for button group

modules/ifc_models/app/assets/stylesheets/ifc_viewer/generic.sass

@@ -8,6 +8,7 @@
 @import "toolbar"
 @import "loading_modal"
 @import "context_menu"
+@import "tooltips"
 
 // The IFC viewer shall fill the viewport to the bottom.
 // This is different to how we style #content else where.
@@ -71,6 +72,7 @@
   margin-right: .5rem
 
 .xeokit-btn
+  position: relative
   background-color: $button--background-color
   vertical-align: middle
   padding: .375rem .75rem

modules/ifc_models/app/assets/stylesheets/ifc_viewer/tooltips.sass

@@ -0,0 +1,11 @@
+@import "content/tooltips"
+
+[data-tippy-content]
+  @extend .tooltip--bottom
+  &:after
+    @extend [data-tooltip]:after
+    content: attr(data-tippy-content)
+    opacity: 1
+
+  &:hover:after
+    visibility: visible