[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/general/res/sass/_main.scss

@@ -65,6 +65,7 @@
 @import "user-environ/frame";
 @import "user-environ/top-bar";
 @import "user-environ/tool-bar";
+@import "user-environ/selecting";
 
 /********************************* VIEWS */
 @import "fixed-position";