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/gui/MainWindow.h

@@ -48,6 +48,8 @@ public:
     MainWindow();
     ~MainWindow();
 
+    QList<DatabaseWidget*> getOpenDatabases();
+
     enum StackedWidgetIndex
     {
         DatabaseTabScreen = 0,