Added new linter that enforces Qt5 SIGNAL/SLOT connection style.
Partial backport of Core PR13529 (adjusted to work with arcanist)
https://github.com/bitcoin/bitcoin/pull/13529/commits/3567b247f43decb6fc102d5b0989d1746fce0441
Depends on D4305
Note for reviewers: D4305 ended up being larger than I realized and is being broken into smaller parts.