[Qt] rename rpcconsole window

- rework window title to not include Bitcoin - in front, as no other
  dialog does this
- use CChainParams::TestNet() for checking testnet, as this is done
  everywhere in Qt code (perhaps we also could remove isTestNet() from
  client model now?)
- favor a connect() call over an own function for clearing the traffic
  graph
- write monospace lowercase (seems to be correct after some web search)
  and add a comment that we should avoid / remove fixed font sizes