[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>

.ci/compile.sh

@@ -149,6 +149,9 @@ if [[ $MAKE_TEST ]]; then
 fi
 if [[ $USE_CCACHE ]]; then
   flags+=("-DUSE_CCACHE=1")
+  # PCH-aware caching is required or ccache refuses to cache any TU that
+  # consumes a precompiled header, silently recompiling everything on every run.
+  ccache --set-config sloppiness=pch_defines,time_macros
   if [[ $CCACHE_SIZE ]]; then
     # note, this setting persists after running the script
     ccache --max-size "$CCACHE_SIZE"