Add network to peers window and peer details

Summary:

net, rpc: change CNodeStats::m_network from string to Network

gui: create GUIUtil::NetworkToQString() utility function

gui: add network column in peers tab/window

gui: rename peer tab column headers, initialize in .h

gui: display network in peer details

This is a backport of core-gui#162

Test Plan:
ninja && src/qt/bitcoin-qt
Check the Peers view: column headers and "Network" field for a selected peer

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10961