[Fronted] Significant refactoring of btn classes

WTD-839
Refactoring in advance of style mods to switcher when in frame;
Moved btn styles from _controls.scss to _buttons.scss;
Markup cleaned up with css classing simplified;

platform/features/plot/res/templates/plot.html

@@ -108,7 +108,7 @@
                         <span class="ui-symbol icon">I</span>
                     </a>
 
-                    <div class="menu-element btn icon-btn very-subtle btn-menu dropdown click-invoke"
+                    <div class="menu-element btn icon-btn s-very-subtle btn-menu dropdown click-invoke"
                          ng-if="plot.getModeOptions().length > 1"
                          ng-controller="ClickAwayController as toggle">