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

src/webui/CMakeLists.txt

@@ -1,38 +1,38 @@
 add_library(qbt_webui STATIC
-# headers
-api/apicontroller.h
-api/apierror.h
-api/appcontroller.h
-api/authcontroller.h
-api/freediskspacechecker.h
-api/isessionmanager.h
-api/logcontroller.h
-api/rsscontroller.h
-api/searchcontroller.h
-api/synccontroller.h
-api/torrentscontroller.h
-api/transfercontroller.h
-api/serialize/serialize_torrent.h
-webapplication.h
-webui.h
+    # headers
+    api/apicontroller.h
+    api/apierror.h
+    api/appcontroller.h
+    api/authcontroller.h
+    api/freediskspacechecker.h
+    api/isessionmanager.h
+    api/logcontroller.h
+    api/rsscontroller.h
+    api/searchcontroller.h
+    api/synccontroller.h
+    api/torrentscontroller.h
+    api/transfercontroller.h
+    api/serialize/serialize_torrent.h
+    webapplication.h
+    webui.h
 
-# sources
-api/apicontroller.cpp
-api/apierror.cpp
-api/appcontroller.cpp
-api/authcontroller.cpp
-api/freediskspacechecker.cpp
-api/logcontroller.cpp
-api/rsscontroller.cpp
-api/searchcontroller.cpp
-api/synccontroller.cpp
-api/torrentscontroller.cpp
-api/transfercontroller.cpp
-api/serialize/serialize_torrent.cpp
-webapplication.cpp
-webui.cpp
+    # sources
+    api/apicontroller.cpp
+    api/apierror.cpp
+    api/appcontroller.cpp
+    api/authcontroller.cpp
+    api/freediskspacechecker.cpp
+    api/logcontroller.cpp
+    api/rsscontroller.cpp
+    api/searchcontroller.cpp
+    api/synccontroller.cpp
+    api/torrentscontroller.cpp
+    api/transfercontroller.cpp
+    api/serialize/serialize_torrent.cpp
+    webapplication.cpp
+    webui.cpp
 )
 
-qbt_target_sources(qBittorrent PRIVATE www/webui.qrc)
+target_sources(qbt_webui INTERFACE www/webui.qrc)
 
-target_link_libraries(qbt_webui PUBLIC qbt_base)
+target_link_libraries(qbt_webui PRIVATE qbt_base)