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/guiconstants.h

@@ -41,4 +41,9 @@ static const int MAX_PAYMENT_REQUEST_SIZE = 50000; // bytes
 /* Number of frames in spinner animation */
 #define SPINNER_FRAMES 35
 
+#define QAPP_ORG_NAME "Bitcoin"
+#define QAPP_ORG_DOMAIN "bitcoin.org"
+#define QAPP_APP_NAME_DEFAULT "Bitcoin-Qt"
+#define QAPP_APP_NAME_TESTNET "Bitcoin-Qt-testnet"
+
 #endif // GUICONSTANTS_H