Improve Search responsiveness

There are two obvious options:
* Use "submit" button for Search field instead of performing search on every text change.
* If the text in the search field changes, don't immediately update list, but schedule an update with a short delay. If the text is changed again (shortly after that), reschedule the update.

The options can be combined.

Current implementation contains hacks that ~~preserve the search field state~~ open new search field with the same text when context menu is recreated, so it should be done carefully.