qt: Make it possible again to specify -testnet in config file

Changes for the datadir chooser have made it impossible to specify
the network (testnet/regtest) in the configuration file for the GUI.

Reorganize the initialization sequence to make this possible again.

- Moves the "datadir" QSetting so that is no longer dependent on the
  network-specific application name (doing otherwise would create a
  chicken-and-egg problem).

- Re-initialize translations after choosing network. There may be a
  different language configured in network-specific settings
  (slim chance, but handle it for sanity).

Fixes point 1 of #3840.

src/qt/intro.cpp

@@ -146,7 +146,7 @@ QString Intro::getDefaultDataDirectory()
     return QString::fromStdString(GetDefaultDataDir().string());
 }
 
-void Intro::pickDataDirectory(bool fIsTestnet)
+void Intro::pickDataDirectory()
 {
     namespace fs = boost::filesystem;
     QSettings settings;
@@ -164,10 +164,7 @@ void Intro::pickDataDirectory(bool fIsTestnet)
         /* If current default data directory does not exist, let the user choose one */
         Intro intro;
         intro.setDataDirectory(dataDir);
-        if (!fIsTestnet)
-            intro.setWindowIcon(QIcon(":icons/bitcoin"));
-        else
-            intro.setWindowIcon(QIcon(":icons/bitcoin_testnet"));
+        intro.setWindowIcon(QIcon(":icons/bitcoin"));
 
         while(true)
         {