[Build] Add precompiled headers for Qt-backed executables

Reparsing QtCore/QtGui/QtWidgets/QtNetwork in ~460 client translation
units is the dominant compilation cost. Precompile the two common layers:
- qtcore_pch.h (Qt Core only; safe even for headless Servatrice)
- qtwidgets_pch.h (adds Gui/Widgets/Network; used by Cockatrice and Oracle)

target_precompile_headers() requires CMake 3.16, now the project minimum.
Estimated 30-50% faster client rebuilds.

servatrice/CMakeLists.txt

@@ -95,6 +95,8 @@ set(DESKTOPDIR
 # Build servatrice binary and link it
 add_executable(servatrice MACOSX_BUNDLE ${servatrice_MOC_SRCS} ${servatrice_RESOURCES_RCC} ${servatrice_SOURCES})
 
+target_precompile_headers(servatrice PRIVATE "${CMAKE_SOURCE_DIR}/cmake/pch/qtcore_pch.h")
+
 if(CMAKE_HOST_SYSTEM MATCHES "FreeBSD")
   target_link_libraries(
     servatrice libcockatrice_deck_list libcockatrice_network_server_remote Threads::Threads ${SERVATRICE_QT_MODULES}