* refactor: const correctness
* refactor: fix some implicit conversions
* refactor: make local pointers const if their objects are not modified
* refactor: do not cast away const in torrent-cell-renderer
* refactor: remove call to deprecated gtk_icon_size_lookup_for_settings
* refactor: member functions that do not mutate their objects should be declared const
* chore: do not end comments with a semicolon
