CMake: overhaul and improve scripts

- Bump minimum required version and
make use of more modern language features
- Rely more on target_...() commands to establish dependency
relationships between targets rather than directory property commands
- Improve libtorrent package discovery
- Enable and handle application features more explicitly
- Improve user-facing output
- Fix various compilation issues on Windows (MSVC and MinGW) and macOS
- Improve handling of translations
- Add explanatory comments where relevant
- Make CMake scripts fully independent of qmake files/details
- Remove old functions/macros

dist/CMakeLists.txt

@@ -1,9 +1,7 @@
-find_package(Qt5Widgets ${requiredQtVersion}) # to conditionally install desktop-related files
-
-if (APPLE)
+if (CMAKE_SYSTEM_NAME STREQUAL "Darwin")
     add_subdirectory(mac)
-elseif (UNIX)
+elseif (UNIX AND (NOT APPLE))
     add_subdirectory(unix)
-elseif (WIN32)
+elseif (CMAKE_SYSTEM_NAME STREQUAL "Windows")
     add_subdirectory(windows)
 endif()