qt: add network-specific style object

Mainly cleanups: Gets rid of isTestNet everywhere, by keeping track
of network-specific theming in a central place.

Also makes GUI no longer dependent on the network ID enumeration, which
alleviates concerns about #4802.

src/qt/networkstyle.h

@@ -0,0 +1,33 @@
+// Copyright (c) 2014 The Bitcoin developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef H_NETWORKSTYLE
+#define H_NETWORKSTYLE
+
+#include <QIcon>
+#include <QPixmap>
+#include <QString>
+
+/* Coin network-specific GUI style information */
+class NetworkStyle
+{
+public:
+    /** Get style associated with provided BIP70 network id, or 0 if not known */
+    static const NetworkStyle *instantiate(const QString &networkId);
+
+    const QString &getAppName() const { return appName; }
+    const QIcon &getAppIcon() const { return appIcon; }
+    const QString &getTitleAddText() const { return titleAddText; }
+    const QPixmap &getSplashImage() const { return splashImage; }
+
+private:
+    NetworkStyle(const QString &appName, const QString &appIcon, const char *titleAddText, const QString &splashImage);
+
+    QString appName;
+    QIcon appIcon;
+    QString titleAddText;
+    QPixmap splashImage;
+};
+
+#endif // H_NETWORKSTYLE