[Frontend] Initial styling for selectable styling

Fixes #1161
New .s-selectable, .s-hover, .s-selected and .s-moveable classes
New supporting color constants
Markup in layouts, fixed position/panels updated
Removed old commented code

platform/commonUI/themes/snow/res/sass/_constants.scss

@@ -63,6 +63,9 @@ $colorInvokeMenu: #fff;
 $colorObjHdrTxt: $colorBodyFg;
 $colorObjHdrIc: pushBack($colorObjHdrTxt, 30%);
 $colorTick: rgba(black, 0.2);
+$colorSelectableSelectedPrimary: $colorKey;
+$colorSelectableSelectedSecondary: pushBack($colorSelectableSelectedPrimary, 20%);
+$colorSelectableHov: $colorSelectableSelectedSecondary;
 
 // Menu colors
 $colorMenuBg: pushBack($colorBodyBg, 10%);