Remove network hardcoded properties from desktop-wallet

[cryptoBeliever]

Current behavior:

Currently, we have in NetworkConfig hardcoded values:

• defaultTestnetNetworkConfig
• defaultMainnetNetworkConfig

Expected behavior:

All network properties should be fetched from the node. Important is to verify offline transactions which may use those hardcoded values too (since we are unable to fetch in offline mode data from the node). In such a case, we have to rebuild the offline transactions view and potentially ask the user to put/choose generation hash / mosaic id (?)