Add drop.less and override drop-z-index variable

less/_import.less

@@ -33,6 +33,7 @@
 
 // JavaScript
 @import "accordion";
+@import "drop";
 @import "dropdown";
 @import "modal";
 @import "slider";

less/drop.less

@@ -0,0 +1 @@
+@drop-z-index: @global-z-index - 19;

less/dropdown.less

@@ -1,4 +1,3 @@
-@dropdown-z-index: @global-z-index - 19;
 @dropdown-min-width: 260px;
 @dropdown-padding: 5px 0;
 @dropdown-background: @global-background;