* refactor: intern repeated strings e.g. downloadDir
Add an app-wide interned QString cache for commonly-repeated
strings. Currently used for download_dir, announce_url, comment,
creator, error_string.
* refactor: keep torrent hash in std::array<char,20>
This replaces the hashString QString allocation with a compile-time
array that's included in sizeof the Torrent struct that owns it.
* refactor: speed up FaviconCache::add() again.
Another iteration on FaviconCache::add() since it's still showing up so
high in my perf tests. add()'s url argument is now a QString instead of
a QUrl, and the class has a private unordered_map that maps QString urls
into Keys. Basically, QUrl generation is so expensive that it's worth
caching the result to avoid constructing the intermediate QUrl object.
Also, ensure that the network reply's `deleteLater()` method is called
so that they don't leak, and add 'svg' to the icon list since it's now
supported on all major browsers.
* refactor: make variant_headers reusable to qt app.
Torrent.cc's `change()` template methods are generically useful to deal
with tr_variant wrangling, but previously were only used in Torrent.cc.
This PR moves them into a new API `VariantHelpers.h` for use by Prefs,
Session, TorrentModel, etc.
