[Frontend] More sanding on label flex-box conversion

open #431
Moved object-label classes into their own include;
Fixing object-label in edit Elements pool and
Inspector; mixin refactoring;

platform/commonUI/general/res/sass/_main.scss

@@ -60,6 +60,7 @@
 @import "overlay/overlay";
 @import "mobile/overlay/overlay";
 @import "tree/tree";
+@import "object-label";
 @import "mobile/tree";
 @import "user-environ/frame";
 @import "user-environ/top-bar";