[Build] Add precompiled headers for Qt-backed executables (#7235)

* [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.

* [Build] Format qtwidgets precompiled header

clang-format include regrouping and a missing trailing newline.

* [Build] Add PCH-aware ccache sloppiness config; format cmake/pch headers

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

format.sh

@@ -18,6 +18,7 @@ include=("cockatrice/src" \
 libcockatrice_* \
 "oracle/src" \
 "servatrice/src" \
+"cmake/pch" \
 "tests")
 exclude=("libcockatrice_rng/libcockatrice/rng/sfmt/" \
 "libcockatrice_utility/libcockatrice/utility/peglib.h" \