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.h

@@ -36,7 +36,7 @@ public:
      * @note do NOT call global GetDataDir() before calling this function, this
      * will cause the wrong path to be cached.
      */
-    static void pickDataDirectory(bool fIsTestnet);
+    static void pickDataDirectory();
 
     /**
      * Determine default data directory for operating system.