Closes#757
Mostly just the UI at this point. Centered the dropdown link and menu, and set up translations. Hooked up the menu click function, and I know we don't usually bother with auto-closing menus, but it felt kinda weird with this one, so I added scroll and click listeners to auto-close.
* caching of search results and downloaded style info for one day
* no prefetching of styles
* only the next search results page is prefetched in unattended fashion
* the "configure" button is shown only after installation
* join the code in one closure, regroup and simplify some functions