Converted selectall.css to less for better import

src/UI/Content/Backgrid/backgrid.less

@@ -1,3 +1,3 @@
 @import "filter";
 @import "paginator";
-@import (css) "selectall.css";
+@import "selectall";

src/UI/Content/Backgrid/selectall.less

@@ -6,7 +6,8 @@
   Licensed under the MIT @license.
 */
 
-.backgrid .select-row-cell,
-.backgrid .select-all-header-cell {
-  text-align: center;
+.backgrid {
+  .select-row-cell, .select-all-header-cell {
+    text-align: center;
+  }
 }