Fix issues reported by clang-tidy readability checks (GTK client) (#4127)

* Fix `readability-container-data-pointer` clang-tidy issues

* Fix `readability-container-size-empty` clang-tidy issues

* Fix `readability-isolate-declaration` clang-tidy issues

* Fix `readability-convert-member-functions-to-static` clang-tidy issues

* Fix `readability-else-after-return` clang-tidy issues

* Fix `readability-redundant-member-init` clang-tidy issues

* Fix `readability-uppercase-literal-suffix` clang-tidy issues

* Fix `readability-make-member-function-const` clang-tidy issues

* Fix `readability-simplify-boolean-expr` clang-tidy issues

* Fix `readability-qualified-auto` clang-tidy issues

* Fix `readability-avoid-const-params-in-decls` clang-tidy issues

* Fix `readability-implicit-bool-conversion` clang-tidy issues

* Fix `readability-inconsistent-declaration-parameter-name` clang-tidy issues

* Add initial .clang-tidy configuration

Currently enabling readability checks only, others to follow. Of the
four disabled readability checks, three (function-cognitive-complexity,
identifier-length, magic-numbers) require extensive refactoring and were
left for later, and one (redundant-access-specifiers) is not planned to
be fixed due to the current coding style - using extra sections to
separate methods from variables of the same visibility.

gtk/Notify.cc

@@ -242,7 +242,7 @@ void gtr_notify_torrent_completed(Glib::RefPtr<Session> const& core, tr_torrent_
         [n](auto& res) { notify_callback(res, n); },
         make_variant_tuple(
             Glib::ustring("Transmission"),
-            0u,
+            0U,
             Glib::ustring("transmission"),
             Glib::ustring(_("Torrent Complete")),
             Glib::ustring(tr_torrentName(tor)),
@@ -276,7 +276,7 @@ void gtr_notify_torrent_added(Glib::RefPtr<Session> const& core, tr_torrent_id_t
         [n](auto& res) { notify_callback(res, n); },
         make_variant_tuple(
             Glib::ustring("Transmission"),
-            0u,
+            0U,
             Glib::ustring("transmission"),
             Glib::ustring(_("Torrent Added")),
             Glib::ustring(tr_torrentName(tor)),