[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/commonUI/general/res/templates/controls/action-button.html

@@ -19,7 +19,7 @@
  this source code distribution or the Licensing information page available
  at runtime from the About dialog for additional information.
 -->
-<a class="t-btn l-btn s-btn s-icon-btn s-very-subtle key-{{parameters.action.getMetadata().key}}"
+<a class="btn s-btn s-icon-btn s-very-subtle key-{{parameters.action.getMetadata().key}}"
    ng-class="{ labeled: parameters.labeled }"
    title="{{parameters.action.getMetadata().description}}"
    ng-click="parameters.action.perform()">