[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.

cmake/pch/qtwidgets_pch.h

@@ -0,0 +1,31 @@
+/** @file qtwidgets_pch.h
+ *  @brief Precompiled header for GUI targets (Cockatrice client, Oracle).
+ *
+ * Includes the Qt Core precompiled header plus the heavy Gui, Widgets and
+ * Network layers that virtually every client translation unit re-parses.
+ * Do not use on Servatrice (headless, QT_DONT_USE_QTGUI).
+ */
+
+#include "qtcore_pch.h"
+
+#include <QAction>
+#include <QApplication>
+#include <QFrame>
+#include <QGraphicsItem>
+#include <QGraphicsScene>
+#include <QGraphicsView>
+#include <QImage>
+#include <QLabel>
+#include <QLayout>
+#include <QMainWindow>
+#include <QMenu>
+#include <QPainter>
+#include <QPushButton>
+#include <QScrollArea>
+#include <QTabWidget>
+#include <QToolBar>
+#include <QTreeWidget>
+#include <QWidget>
+
+#include <QNetworkAccessManager>
+#include <QNetworkReply>