clazy: Convert many old-style connects into function pointer based

Since Qt5, signal/slot connections can be expressed using function
pointers rather than the SIGNAL/SLOT macros. This is a) much more
efficient, and b) provides a compile-time check for the sender
and receiver being compatible.

Let clazy auto-fix old-style connects where it can. However, a lot of
occurrences remain where we'll need manual intervention for one
reason or another.