chore: fix clang-tidy-11 warnings (#1436)

* refactor: mark subclass' destructors as override.

* refactor: use QUrl to parse announce URL strings.

The prompt for this was to work around a clang-tidy issue where
"char* host = nullptr;" triggers a "don't use varargs" warning,
but on the other hand it's also terser / cleaner.

* refactor: make the TorrentDelegate brushes const.

* refactor: s/auto/auto const*/ where appropriate

* chore: add some nonconst global warning exemptions

* chore: turn off warnings in GTest

* refactor: just disable the clang-tidy warning.

Apparently a std::array<T>::iterator is a T* on clang, since clang-tidy's
readability warning says we should use 'auto*' instead of 'auto'. However
adding that annotation fails on MSVC, where the is apparently _not_ a raw
pointer.

Since there's not a way to satisfy both of them at the same time, disable
the warning.

qt/FileTreeModel.h

@@ -45,7 +45,7 @@ public:
 
 public:
     FileTreeModel(QObject* parent = nullptr, bool is_editable = true);
-    ~FileTreeModel();
+    ~FileTreeModel() override;
 
     void setEditable(bool editable);