Add support for multiple testnets

[cryptoBeliever]

Current behavior

Currently, the desktop wallet supports one testnet. Network generation hash is a part of the wallet profile. Generation hash is always taken from the official testnet. A switch to another testnet (with a different generation hash) will cause the error/warning and be asked to create a new profile.

The error/warning looks like that:

If the user ignores the warning it will be allowed to use a different network.

Expected behavior

The wallet should allow users to support multiple testnets (with different generation hashes).

Something to consider / problems:

1. Should the wallet profile save the network generation hash and show a warning when connecting to the node with a different one (but the same network type)? Maybe it should be done like in the mobile wallet that will allow the switch to a node with the same network type but a different generation hash? I think we should allow transparently switch between testnet networks (without error - maybe just some warning/notification).

2. Should the statistics service support multiple testnets as well? Because the community will probably create networks dynamically and statistics services will be replaced with the node watch maybe a better idea is just to do changes on the wallet side.

We can allow the user to add testnet nodes from different networks and add grouping in the node list by generation hash. Something like this:

Testnet 1 (<generation hash1>)
- node1
- node2
- node3

Testnet 2 (<generation hash2>)
- node4
- node5
- node6

The exact solution is to discuss when the task will be taken. Also, current code analyses may be needed.