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.