* perf: faster file tree building in GTK client
* perf: faster GTK client FileList
* use a std::map for faster node lookup when building a file tree
* use tr_get_mime_type_for_filename() for mime-type lookup
* remove unnecessary layer of indirection when building the
keys for lookuops in the mime-type icon cache
* be more careful to only call operator= on proxies whose values
have actually changed, since that assignment is expensive
* refactor: make this variable const
* refactor: replace redundant type with auto
* refactor: use the nullptr literal
* refactor: replace declaration by structured binding declaration
* refactor: include <memory> when using shared_ptr or unique_ptr
* refactor: include <cstdio> iff we use it
* refactor: include <cstring> iff we use it
* refactor: include <cstdlib> iff we use it
* refactor: include <string_view> or <string> iff we use it
* refactor: include <array> iff we use it
* refactor: include <ctime> iff we use it
* refactor: include <cctype> iff we use it
* refactor: misc #include cleanups in libtransmission
* (C++) Macros should not be used to define constants
* (C++) Memory should not be managed manually
* (C++) "void*" should not be used in typedefs, member variables, function parameters or return type
* (C++) When the "Rule-of-Zero" is not applicable, the "Rule-of-Five" should be followed
* (C++) "switch" statements should have "default" clauses
* (C++) "explicit" should be used on single-parameter constructors and conversiosn operators
* (C++) Non-const global variables should not be used
