feat(CSS): base styling for panel elements

* ensure that basic LightDOM styles are applied to panel children
  * hx-panel-head
  * hx-panel-body
  * hx-panel-foot

Author: Ryan A. Johnson

src/helix-ui.less

@@ -60,6 +60,9 @@ figure {
 @import 'core/hx-modalfoot';
 @import 'core/hx-modalhead';
 @import 'core/hx-panel';
+@import 'core/hx-panel-body';
+@import 'core/hx-panel-foot';
+@import 'core/hx-panel-head';
 @import 'core/hx-popover';
 @import 'core/hx-popover-body';
 @import 'core/hx-popover-foot';

src/helix-ui/styles/core/hx-panel-body.less

@@ -0,0 +1,4 @@
+hx-panelbody, // DEPRECATED, remove in v1.0.0
+hx-panel-body {
+  display: block;
+}

src/helix-ui/styles/core/hx-panel-foot.less

@@ -0,0 +1,4 @@
+hx-panelfoot, // DEPRECATED, remove in v1.0.0
+hx-panel-foot {
+  display: block;
+}

src/helix-ui/styles/core/hx-panel-head.less

@@ -0,0 +1,4 @@
+hx-panelhead, // DEPRECATED, remove in v1.0.0
+hx-panel-head {
+  display: block;
+}