Complete refactor of Browser Integration classes

* Removed option to attach KeePassXC to the browser extension. Users must use the proxy application to communicate with KeePassXC.
* Significantly streamlined proxy code. Used same implementation of stdin/stdout interface across all platforms.
* Moved browser service entry point to BrowserService class instead of NativeMessagingHost. BrowserService now coordinates the communication to/from clients.
* Moved settings page definition out of MainWindow
* Decoupled BrowserService from DatabaseTabWidget
* Reduced complexity of various functions and cleaned the ABI (public vs private).
* Eliminated BrowserClients class, moved functionality into the BrowserService
* Renamed HostInstaller to NativeMessageInstaller and renamed NativeMessageHost to BrowserHost.
* Recognize XDG_CONFIG_HOME when installing native message file on Linux. Fix #4121 and fix #4123.

src/browser/BrowserShared.cpp

@@ -0,0 +1,46 @@
+/*
+ *  Copyright (C) 2020 KeePassXC Team <team@keepassxc.org>
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "BrowserShared.h"
+#include "config-keepassx.h"
+
+#include <QCoreApplication>
+#include <QProcessEnvironment>
+#include <QStandardPaths>
+#include <QVariant>
+
+namespace BrowserShared
+{
+    QString localServerPath()
+    {
+        const auto appName = qApp->property("KPXC_QUALIFIED_APPNAME").toString();
+        const auto serverName = QStringLiteral("/%1.BrowserServer").arg(appName);
+#if defined(KEEPASSXC_DIST_SNAP)
+        return QProcessEnvironment::systemEnvironment().value("SNAP_USER_COMMON") + serverName;
+#elif defined(Q_OS_UNIX) && !defined(Q_OS_MACOS)
+        // Use XDG_RUNTIME_DIR instead of /tmp if it's available
+        QString path = QStandardPaths::writableLocation(QStandardPaths::RuntimeLocation);
+        return path.isEmpty() ? QStandardPaths::writableLocation(QStandardPaths::TempLocation) + serverName
+                              : path + serverName;
+#elif defined(Q_OS_WIN)
+        // Windows uses named pipes
+        return serverName;
+#else // Q_OS_MACOS and others
+        return QStandardPaths::writableLocation(QStandardPaths::TempLocation) + serverName;
+#endif
+    }
+} // namespace BrowserShared