Now that the properties are loaded immediately when the selected torrent
or the property tab is changed, the frequency of the updates can be
reduced without affecting the user experience.
New update intervals:
* General: 5 seconds (10 if qBT is not reachable)
* Tracker list: 10 seconds (20 if qBT is not reachable)
* File list: 5 seconds (10 if qBT is not reachable)
Don't wait for new data to be fetched, request them as soon as
they are required, i.e. when a property tab is loaded or when a
different torrent is selected.
Don't reload HTML on tab change. Load the entire structure once and
keep the unused bits hidden.
Keep also the JS code separately, one file per tab.
NOTE: tabs content is not loaded. This will be fixed in the next commit.
