Files app: Ignores pointer events on buttons on dialog footer.

We should ignore pointer events on disabled buttons.
It prevents unexpected command dispatches, and it is consistent with paper-button.

BUG=632270
TEST=manually

Review-Url: https://codereview.chromium.org/2256143002
Cr-Commit-Position: refs/heads/master@{#412812}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -938,6 +938,11 @@ html[dir='rtl'] #search-box .clear {
   z-index: 0;
 }
 
+.dialog-footer button[disabled] {
+  cursor: auto;
+  pointer-events: none;
+}
+
 .dialog-footer button {
   font-weight: 500;
 }