Add Spanish locale.

UI improvement:
Remove "Webextension" from title

Display "no matching results" when there's no search results:
https://github.com/program-in-chinese/HistoryInThreads_WebExtension/issues/17

Instead of been displayed in a separate column, the visit date & time is now the title of each link, and can be seen if you hover mouse on a link.
Based on user feedback: https://github.com/program-in-chinese/HistoryInThreads_WebExtension/issues/10

Small improvement for https://github.com/program-in-chinese/HistoryInThreads_WebExtension/issues/14: show url when title of the web page is empty string

This is a version with major refactoring and there are some observable differences in UI and feature.

1. icons of the pages won't be available, caused by an obsolete API.
2. the visite time of each page is shown in a separate column. Unless it causes too much trouble, I intend to leave it there without providing an option to hide it.
3. it takes much longer than the old version to search. Especially, the first time you select a long history range in one session, like 'this year', it will take a long time to index the history visits. The later searches using the same history range won't need indexing again. The main reason is change of extension API. Details please see https://discourse.mozilla.org/t/api-to-fetch-visits-by-multiple-urls/18467.
4. the search bar doesn't always stay on top, and when you scrolls down, it goes up and out of sidebar. You need to scroll back to access it.
5. context menu is now the same as that of a normal link in any web page, instead of a customized one.